Copyright 0 1984 by the Genetics Society of America

THERMAL RESCUE OF UV-IRRADIATED BACTERIOPHAGE T 4 AND BIPHASIC MODE OF ACTION OF THE WXY

SYSTEM

MARK A. CONKLING AND JOHN W. DRAKE’

Laboratory of Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709

Manuscript received October 14, 1983 Revised copy accepted March 22, 1984

ABSTRACT

When ultraviolet-irradiated bacteriophage T4 is assayed at plating temper- atures ranging from 20” to 40°, its survival increases at the higher tempera- tures. This “thermal rescue” requires an intact WXY system but not the denV pyrimidine dimer excision system. Mutation rates decrease with increasing tem- perature, indicating that some lesions processed in a mutagenic manner at lower temperatures are accurately repaired or circumvented at high tempera- tures. When both the cold sensitivity of UV survival in the wild type and the temperature sensitivity of newly isolated ts mutants of uusX and uusY were used, expression of the WXY system was monitored in temperature shift UV survival experiments and was found to be biphasic: the uusX and uusY functions increase UV survival in two increments, one at an early and another at a late stage of infection. The UVSW function, however, increases UV survival only early in infection.

HE bacteriophage T 4 WXY system was originally defined by mutations (of T genes now called uvsW, uvsX and uvsY) that decrease recombination fre- quencies and UV survival and act independently of the denV pyrimidine dimer excision system. It gradually became clear that these genes also affect survival after treatments with a variety of physical and chemical agents and, in addition, affect mutagenesis, recombination, DNA synthesis and suppression of gene 49 defects (BERNSTEIN and WALLACE 1983). The WXY system has, therefore, usu- ally been termed an error-prone repair system, a designation that certainly denotes only some of its function and that may be mechanistically misleading.

Having selected conditional alleles of the WXY system by their ability to suppress gene 49 mutations (CONKLING and DRAKE 1984), we set out to mea- sure the timing of their effects upon UV survival. To our surprise, UV-irra- diated wild-type T 4 exhibited substantially higher survivals at higher temper- atures. We call this effect thermal rescue and show that it depends upon the WXY system.

Both the cold sensitivity of the wild-type and the heat sensitivity of the new ts mutants were then used in temperature shift experiments to determine the timing of action of the WXY system upon UV survival. The uvsX and uvsY

’ To whom communications should be addressed.

Genetics 107: 525-536, August, 1984.

526 M. A. CONKLING AND J. W. DRAKE

genes together determine two incremental effects upon UV survival, the first during the initial minutes after infection and the second much later. In con- trast, the UVSW gene affects only the early step.

MATERIALS AND METHODS

Bacterial and phage strains, media and general methods for measuring survival and mutation and for irradiation are described in CONKLING and DRAKE (1984).

Ionizing radiation treatments: The ionizing radiation sources differed from that used in the preceding paper. Most exposures were in air from a 50-kVP Picker X-ray machine using a beryl- lium window and a Machlett OEG-50 tube, filtered through 50 mg/cm* of aluminum. The dose rate determined by ferrous sulfate dosimetry and corrected for atomic number difference from water was about 6 krad/min (E. C. POLLARD, personal communication). However, one set of exposures, as noted in RESULTS, used a Shepherd model 431 '"Cs y source at a dose rate of about 12 krad/min to a rotating sample under 70 psi of nitrogen but without prior nitrogen flushing of the 5-ml samples.

Temperature shqt experiments: Unirradiated or UV-irradiated T 4 particles were adsorbed to log- phase BB or CR63 cells at 5 X 108/ml in L broth at 0" for 7-10 min at multiplicities less than 0.1 particles per cell. At time zero the phagecell complexes were diluted 50-fold into L broth at the first temperature (the "permissive" temperature, e.g., that giving the higher survival, depending upon whether the repair system was cold sensitive or heat sensitive). At various times thereafter, samples were diluted an additional 20- to 50-fold in L broth at both the permissive and the nonpermissive temperatures, the latter being that producing the lower survival. At 30 min the complexes were assayed for plaque-forming units, the plates being incubated at the second tem- perature for 2 hr and then shifted to 37". Control experiments confirmed that there were no detectable bursts during the first 30 min, and that the final shift to 37" did not affect survivals.

Induction of the SOS response: The X-sensitive E. coli K12 strain CR63 was grown to midlog phase in L broth, centrifuged, resuspended in M9S at 109/ml, UV irradiated at a dose rate of about 3 J/m* sec, diluted ten-fold into L broth and incubated for 30 min at 37°C on a rotary shaker to achieve expression of SOS functions (WITKIN 1975). Wild-type X was UV irradiated to a survival of about 0.001 and was adsorbed to uninduced and SOS-induced cells at low multiplic- ities. The complexes were then diluted and plated to determine survivals of plaque-forming units. Maximum Weigle reactivation, about 50-fold, was obtained after 30 sec of irradiation to the host cells, and this dose was adopted for the subsequent experiments with T4. T o determine the effect of SOS induction upon survival and mutagenesis of T4, uninduced and SOS-induced cells were infected with T 4 at multiplicities less than 0.1 (and in the absence of X particles), and the complexes were then plated on CR63 cells to determine both survivals and r + rl mutation rates (CONKLING and DRAKE 1984).

RESULTS

Thermal rescue The preceding paper (CONKLING and DRAKE 1984) described the isolation

and properties of heat-sensitive ts mutants of the UVSX and UVSY genes. During the course of that study, we noticed that the survival of UV-irradiated wild- type T 4 particles varied with plating temperature: higher survivals were ob- tained at higher temperatures.

Figure 1A describes the effects of incubation temperature upon the UV inactivation kinetics of wild-type T4B, using the plating protocol described by CONKLING and DRAKE (1984) in which the plates are first incubated for 2 hr at the prescribed temperature and then shifted to 37". The back-extrapolate of the survival curves remained approximately constant at about 1.8, whereas

MODE OF T4 WXY ACTION 527

their terminal slopes at 42" and 20" were in the ratio of about -3:-4. Ther- mal rescue occurs to the same extent in T4B and T4D, as judged from a single UV dose producing survivals ranging from 0.002 to 0.008 (data not shown); thus, thermal rescue is not strain specific.

An alternative way to present the thermal rescue data is to plot relative survival, the survival at a temperature T relative to the survival at 20" for each UV dose. The data of Figure 1A are replotted in this manner in Figure 1B. There is no increase in titer with temperature in unirradiated ("0 sec") particles. The increases in relative survival of irradiated particles are approxi- mately linear with increasing temperatures. Relative survival plots will hence- forth be used to display the effects of various repair defects upon thermal rescue, but the further kinetic analysis of these plots will be postponed to the

To discover the genetic determinants of thermal rescue, the experiments underlying Figure 1 were repeated with the mutations v in UVSV, m33 in uvsW, p x in uvsX and y in uvsY. The corresponding relative survival curves are shown in Figure 2. The uvsV mutation had no effect on thermal rescue other than that of a simple UV dose reduction factor. The uvsW mutation permitted substantial thermal rescue but, as we shall show later, altered the value of an important kinetic parameter. The uvsX and uvsY mutations abolished thermal rescue. These results indicate that thermal rescue is the result of cold sensitivity within the WXY system and not within the uvsV-encoded pyrimidine dimer excision system.

DISCUSSION.

528 M. A. CONKLING AND J. W. DRAKE

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TEMPERATURE ('Cl FIGURE 2.-Thermal rescue in repair-defective T 4 mutants. A: Lack of thermal rescue in the

uusX mutant pr. B: Lack of thermal rescue in the uusY mutant y. C: Thermal rescue in the uusV mutant U. D: Thermal rescue in the uusW mutant m33.

We next inquired whether UV-induced mutagenesis in wild-type T 4 was subject to thermal rescue. The protocols for measuring UV mutagenesis at different temperatures were those described by CONKLINC and DRAKE (1 984), and the r+ + r mutation rate was measured on B cells at 20" and 42" (Figure 3). The induced mutation rate was between 1.8- and 2-fold higher at the lower temperature, and reconstruction controls demonstrated that this difference was not an artifact of the efficiency of mutant detection. Thus, thermal rescue is antimutagenic, apparently by a factor somewhat larger than its 1.3-fold effect on inactivation.

Since the WXY system affects survival and mutagenesis after ionizing as well as UV irradiation, we measured the extent of thermal rescue after X and y irradiation. Figure 4 reveals the absence of a temperature effect on survival after X irradiation. However, when a single 540-krad dose was delivered to T4B from a 13'Cs source, small but consistent amounts of thermal rescue were detected, provided that the "42"" plates were prewarmed to 30" before use and were placed at 42" immediately after the top agar layer had hardened. For instance, the average survival was 3.7 X at 23" and 5.5 X lop5 at 42". This corresponds to only about a 4% change in slope, compared with the approximately 33% change observed with UV. In contrast to this very small effect on inactivation, two determinations of the r+ + r mutation rate in the same 7-irradiated population revealed a much larger effect on mutagenesis.

MODE OF T4 WXY ACTION 529

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FIGURE 3.-Thermal reduction of UV mutagenesis in T4B plated on B cells. The error bars are based solely on the counts o f r mutants and represent single standard deviations.

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FIGURE 4.-Lack of thermal rescue of survival in X-irradiated T4B. Open circles: 22". Closed circles: 42". Note that the open and closed circles uniformly overlap.

530 M. A. CONKLING AND J. W. DRAKE

The average mutant frequency was 0.043% in the unirradiated controls, 0.40% at 23" and 0.28% at 42"; thus, mutation rates were approximately 50% greater at the lower temperature. The combined results of UV, X-ray and y- ray inactivation experiments demonstrate that thermal rescue of survival de- pends strongly upon the type of lethal damage, and that thermal antimuta- genesis can still be observed when rescue of survival is almost absent (but see DISCUSSION).

Since uvsX and uvsY mutations are efficient suppressors of gene 49 defects (see CONKLING and DRAKE 1984 and references therein), we measured the plating efficiency of the gene 49 amber mutant amE727x1 on amber-nonsup- pressing host cells as a function of temperature. The plating efficiency of the amber mutant, relative to that of the wild type, was 1.7 X at 20" and 1.6 X at 42", indicating that the gene 49 defect was neither cold nor heat sensitive.

Timing of WXY infuence upon W survival We now possess two classes of temperature-sensitive alleles of the WXY sys-

tem: ts mutants of the uvsX and uvsY genes (CONKLING and DRAKE 1984) and the cold-sensitive wild type. When either are used, it becomes possible to estimate the time during the cycle of infection when the WXY system exerts its influence upon UV survival. The general form of the test is the classical T 4 temperature shift experiment. Irradiated or unirradiated particles are preadsorbed to cells on ice so that no development ensues. The complexes are then transferred at time zero to a temperature that is permissive in the sense that it produces the higher UV irradiation survival (42" for the wild type, 30" for the ts mutants). At intervals thereafter, the complexes are shifted to a temperature that is nonpermissive in the sense that it produces the lower survival (22" for the wild type, 42" for the ts mutants). The time course of the increase in survival during tenure at the permissive temperature then provides a profile of the time course of WXY action.

The time course of WXY action in the wild type is shown in Figure 5A, where survivals of samples shifted from 42" to 22" are plotted relative to the titers of samples pipetted from 42" to 42". (There is no effect of temperature upon unirradiated T 4 in these experiments.) The irradiated sample displays not one, but two, increments of survival with time at the permissive tempera- ture. The first increment is typically complete by about 5 min, whereas the second typically occurs during the interval from 15 to 20 min. The two incre- ments are of approximately equal magnitude whether expressed as percent of the total increment or as percent of the total reduction in lethal hits.

Since thermal rescue is retained to a considerable extent in a uvsW mutant, it becomes possible to examine the time of expression of the residual umX+ and uvsY+ functions. Figure 5B shows the results using the uvsW mutant m33. Here the early increment of survival is missing, nearly all of the recovery occurring in the interval from 15 to 30 min after infection. These two sets of results, one in the wild type and the other in m33, suggest that the first increment of survival seen in the wild type is determined jointly by all three genes, whereas the second increment does not require the uvsW+ function.

MODE OF T4 WXY ACTION 531

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0.8

0.6

0.4

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TIME (MINI OF TEMPERATURE SHIFT FIGURE 6.-Time course of uvsX and uusY gene influences on survival determined by temper-

ature shift experiments. The experiments are analogous to those shown in Figure 5 except that the direction of the temperature shift is reversed because the systems are heat sensitive rather than cold sensitive. Since the ts mutations reside in an amE727x1 (gene 497 background, amber- suppressing CR63 host cells were used to suppress the gene 49 amber mutation. Open circles: no UV. Closed circles: 80 sec UV.

More direct analyses of the time of expression of the UVSX and uvsY genes were possible using their ts alleles and reversing the direction of the temper- ature shifts. Typical results are shown in Figure 6B for a ts allele of uvsY. The results are remarkably similar to those obtained using cold sensitivity, despite the 12 O difference in permissive temperatures. Both ts mutants displayed early increments of survival that were largely completed by 5 min and late incre-

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FIGURE '].-Lack of effect of host SOS system upon T4 UV inactivation and mutagenesis. These tests were performed using CR63 cells at 37"; open symbols indicate noninduced cells, and closed symbols indicate SOS-induced cells (see MATERIALS AND METHODS).

ments of similar magnitude that occurred mostly during the interval from 15 to 20 min. (Similar kinetics were observed using other UV doses.) These results confirm the conclusion that the uvsX and uvsY genes are required for both increments of survival.

Influence of host on survival and mutagenesis

Many T 4 workers have observed that T 4 exhibits indistinguishable UV- induced inactivation kinetics on isogenic recA' and recA- hosts (e.g., MORTEL- MANS and FRIEDBERC 1972). Although these results suggest that the survival of UV-irradiated T 4 is independent of the host recA system, this conclusion is limited by the likelihood that the induction of the SOS system cannot occur after T 4 infection, and that constitutive SOS levels are insufficient to produce a detectable effect. In addition, any host effect might be concealed by the action of T4's own WXY system. T o address these possibilities, the kinetics of UV-induced inactivation and mutagenesis were examined using wild-type T 4 and a uvsY mutant in both uninduced and SOS-preinduced host cells (Figure 7). N o significant differences in either inactivation or mutation rates were detected between induced and uninduced cells. These results support the gen- eral conclusion that neither survival nor mutagenesis in T4 depends signifi- cantly upon the functional state of the host recA system, although other aspects of T 4 DNA metabolism may occasionally be affected (e.g, PRIEMER and CHAN 1978).

During the course of these and previous studies we noticed that UV-irradi-

MODE OF T4 WXY ACTION 533

ated T 4 particles sometimes show small but consistent differences in survival when plated on B and K12 strains of E. coli. Within a given T 4 genotype, these differences are maintained between 30" and 42". There is little effect with the wild type, the terminal slope being the same on KB and BB cells in one set of experiments and about 5% higher on CR63 than on BB cells in another set of experiments. The terminal slopes of T4D x and T4B p x were 9 to 13% higher on KB or CR63 cells than on BB cells. The terminal slope of T4B y was about 18% higher on KB than on BB cells. These effects are large enough to render scoring of uvsX and uvsY mutants difficult when switch- ing between B and K12 strains unless appropriate care is taken. These effects are independent of temperature and are quantitatively significant only in the case of uvsX and uvsY mutants. Their basis is obscure.

DISCUSSION

The discovery of thermal rescue, the enhanced survival of UV-irradiated T 4 at higher temperatures, has occurred remarkably late when one considers the many investigators who have UV irradiated T 4 in a vast number of circum- stances since the middle 1940s. The possibility that the cold sensitivity of the WXY system is due to a recent mutation is faulted by the demonstration of thermal rescue in both T4B and T4D, these strains having existed separately for several decades.

The involvement of the WXY system in thermal rescue is deduced from the lack of cold-sensitive UV survival in uvsX and uvsY mutants and from the intermediate degree of cold sensitivity in a uvsW mutant (see also below). In addition, another major WXY function, UV mutagenesis, is temperature sensi- tive in the wild type. In contrast, the denV pyrimidine dimer excision system, which acts independently of the WXY system, is not involved in thermal rescue: a denV- mutant was as cold sensitive for UV survival as was the wild type.

The temperature sensitivity of the WXY system is unusual. Temperature- sensitive mutants often exhibit a narrow transition temperature range between their functional and nonfunctional states, the transition being sigmoidal in character. As Figures 1 and 2 reveal, however, the survival of UV-irradiated T 4 is an approximately linear function of temperature over the entire range from 22" to 42". It therefore seems unlikely that the WXY system harbors a discrete cs mutation. Instead, we suspect that this linear response reflects some other temperature-dependent factor. Perhaps the rate-limiting step in the for- mation or further processing of the anticipated main product of the WXY system, namely, recombining DNA, is linearly temperature dependent. Can- didates for such a step might include the amounts and configurations of single- stranded DNA, the viscosity- and diffusion-limited annealing of complementary strands, the rates of branch migration and the rates of incorporation of DNA into phage heads.

Thermal rescue from UV damage also affects mutagenesis: both mutational and lethal hits are reduced at higher temperatures. Therefore, both the error- prone and the error-proof components of the wild-type WXY repair system

534 M. A. CONKLING AND J. W. DRAKE

FIGURE 8.-Transformation of the kinetics of thermal rescue. The slopes of the lines of Figures 1B and 2B are plotted as functions of UV dose on logarithmic coordinates. The reference line in panel B has unit slope.

seem to operate more effectively at higher temperatures and by similar factors. (However, the ratio of the two does change slightly, from about 8.2 X at 20" to about 5.4 X at 42", these values being calculated from the data of Figures 1 and 3-the lethal hits from the linear portions of the survival curves and the mutational hits from the pathway r+ + r.)

Thermal rescue depends strongly upon the nature of the damage. For in- stance, the survival of particles inactivated by ionizing radiations is independent of temperature (Figure 4) except for a very small effect with y as opposed to X irradiation. This temperature indifference probably reflects the mode of killing by ionizing radiations, which often produce lethal double-strand breaks (e.g., FREIFELDER, DONTA and GOLDSTEIN 1972); such a genome could rarely be restored by recombinational repair under conditions of single infection (see HANAWALT et al. 1979). In contrast to survival, however, y-induced mutagen- esis is reduced at a high temperature by a factor (1.5-fold) that is similar to that (1.9-fold) seen with UV-induced mutagenesis.

We have already remarked the linearity of thermal rescue expressed as a function of temperature. When the slopes of such curves are plotted as func- tions of UV dose on logarithmic coordinates (Figure 8), straight lines are obtained whose slopes reveal the exponent of the UV dose dependency of thermal rescue. The wild-type curve exhibits a two-hit character, the slope in Figure 8A (calculated from the data of Figure 1 B) being 2.05 with a coefficient of determination (r') of 0.97; two independent measurements yielded slopes of 2.04 (r' = 0.95) and 2.12 (r' = 0.98). The corresponding uusV curve (not shown) yielded a slope of 2.08 (r' = 0.94); when dose was normalized to lethal hits, its dose dependency coincided with that of the wild type, further sup- porting our conclusion that the denV system is not involved in thermal rescue. The uusW curve (Figure 8B, calculated from the data of Figure 2D), on the

MODE OF T4 WXY ACTION 535

other hand, yielded slopes in independent determinations of 1.27 (r2 = 0.95), 1.36 (r2 = 0.96) and 1.13 (r2 = 0.98), for an average of 1.25. Thus, thermal rescue in the uvsW mutant exhibits a UV dose dependency that is clearly greater than single-hit but also clearly less than two-hit. Therefore, thermal rescue in a uvsW mutant is quantitatively distinct from that in the wild type, a point to which we shall return after considering the temperature shift experi- ments.

T4 genes are typically transcribed at one or more temporal phases, such as “immediate early,” “early” or “late” (see BRODY, RABUSSAY and HALL 1983). Temperature-dependent mutants can be used in temperature shift experiments to determine the period during which a gene must act in order for a vital function to result in viable progeny. Both the cold sensitivity of the wild type and the heat sensitivity of uvsX and uvsY mutants were used in temperature shift experiments to determine when the WXY system exerts its effects upon the survival of UV-irradiated particles. Despite the different temperatures used in these reciprocal experiments, each revealed the same pattern, a biphasic increase in survival. We are not aware of any other example of such a biphasic response in a discrete phenotypic endpoint like UV survival. A good case has been made by MELAMEDE and WALLACE (1977) for the early transcription of uvsX and uvsY within 2 min (thus placing them in the “immediate early” cat- egory) and for their translation within the first 8 min. Their further observa- tion, that an increase in T4 DNA synthesis that is sensitive to mitomycin C occurs by 15 min (at 37”) but requires prior protein synthesis before 8 min, is of particular interest because of its temporal correlation with the second phase of thermal rescue.

Whereas uvsX and uvsY mutations each abolished both phases of thermal rescue, a uvsW mutation abolished only the first component. Patterns of DNA metabolism also differ between uvsW and uvsX or uvsY mutants. In a uvsW mutant, DNA synthesis proceeds normally but packaging is reduced (HAMLETT and BERGER 1975). In uvsX and uvsY mutants, DNA synthesis begins normally but falls off sharply after 12 min (CUNNINGHAM and BERGER 1977; MELAMEDE and WALLACE 1977). It is unclear, however, whether these differences reflect different roles for uvsW and uvsX or uvsY in the initial phase of thermal rescue.

It is tempting to relate the action of uvsX and uvsY in two distinct stepwise components of survival to the two-hit character of the UV dose dependency of thermal rescue and the action of uvsW in but one component of incremental survival to the 1.25-hit character of its UV dose dependency of thermal rescue. However, the correlation is numerically imperfect and lacks independent s u p port. It may thus be illusory. On the other hand, it could reflect distinct phases in WXY processing of UV-induced lesions, such as the early development of a capacity for damage-bypass DNA synthesis and the late development of a ca- pacity for recombinational repair, or the early development of one mode of recombinational repair and the late development of a second mode. The bio- chemical characterization of the WXY system is as yet insufficient to offer much help in distinguishing among such possibilities.

In conclusion, we have used both the cold sensitivity of the wild-type and the heat sensitivity of temperature-sensitive uvsX and uvsY mutations to dem-

536 M. A. CONKLING AND J. W. DRAKE

onstrate the two-step nature of WXY action on potentially lethal UV damage. Ongoing biochemical studies in several laboratories now offer the best hope for revealing the nature of this WXY action.

It is a pleasure to acknowledge numerous critical discussions with our colleague LYNN RIPLEY. We also thank ERNEST POLLARD for the use of his X-ray source.

LITERATURE CITED

BERNSTEIN, C. and S. S. WALLACE, 1983 DNA repair. pp. 138-151. In: Bacteriophage T4, Edited by K. C. MATHEWS, E. M. KUTTER, G. MOSIG and P. B. BERGET. American Society for Microbiology, Washington, D.C.

Regulation of transcription of prereplicative genes. pp. 174-183. In: Bacteriophage T4, Edited by K. C. MATHEWS, E. M. KUTTER, G. MOSIG and P. B. BERCET. American Society for Microbiology, Washington, D.C.

Isolation and characterization of conditional alleles of bacteriophage T4 genes UUSX and UVSY. Genetics 107: 505-523.

riophage T4. 1. Pathway analysis. Virology 8 0 67-82.

of Opedependent damage in single- and double-strand DNA phages. Virology 50: 5 16-5 19.

DNA in bacteriophage T4. Virology 63: 539-567.

and mammalian cells. Annu. Rev. Biochem. 48: 783-836.

BRODY, E., D. RABUSSAY and D. H. HALL, 1983

CONKLING, M. A. and J. W. DRAKE, 1984

CUNNINGHAM, R. P. and H. BERGER, 1977

FREIFELDER, D., S. T. DONTA and R. GOLDSTEIN, 1972

HAMLETT, N. R. and H. BERCER, 1975

HANAWALT, P. C., P. K. COOPER, A. K. GANE~ON and C. A. SMITH, 1979

MELAMEDE, R. J. and S. S. WALLACE, 1977

MORTELMANS, K. and E. C. FRIEDBERG, 1972

Mutations affecting genetic recombination in bacte-

X-ray inactivation of bacteriophages: role

Mutations altering genetic recombination and repair of

DNA repair in bacteria

Properties of the nonlethal recombinational repair x

Deoxyribonucleic acid repair in bacteriophage T4:

The effects of virus and host genes on recombination

and y mutants of bacteriophage T4. 11. DNA synthesis. J. Virol. 2 4 28-40.

observations on the roles of the x and y genes and of host factors. J. Virol. 10: 730-736.

among ultraviolet-irradiated bacteriophage T4. Virology 8 8 338-347.

Persistence and decay of thermoinducible error-prone repair activity in nonfilamentous derivatives of t i f l Escherichia coli B/r: the timing of some critical events in ultraviolet mutagenesis. Mol. Gen. Genet. 142: 87-103.

Corresponding editor: G. MOSIG

PRIEMER, M. M. and V. L. CHAN, 1978

WITKIN, E. M., 1975