REVIEWS it demonstrated hydrolysis of two XTP molecules per elongation cycle 27. Also, the stoichiometry of the aa-tRNA inter- action with EF-Tu-GTP recently became controversial, in many experiments, there is a 1 : 1 : 1 stoichiometry of the aa-tRNA: EF-Tu : GTP. However, a change in the reaction conditions led to a stoichiometry of 1 : 2 : 2 (Ref. 28). is a pentameric aa-tRNA-EF-Tu2~TP ~ com- plex a physiological one? The structure of the ternary complex as shown in Fig. 6 considers only one aa-tRNA for EF- Tu~TP, and there is no obvious need for a second EF-Tu-GTP to interact with the same aa-tRNA. Therefore, if a sec- ond EF-Tu-GTP participates in a pen- tameric complex, it will probably also contribute its own tRNA-binding site. Its possible role could be, for example, interaction with peptidyl-tPaNA. Jdthough the interaction of peptidyl-tRNA with EF-Tu-GTP in solution is rather weak, it was not excluded experimentally that such an interaction takes place on ribo- somes. A coordinated step involving two EF-Tu-GTPs fixing one aminoacyl- and one peptidyl-tRNA each to their respective A- and P- (peptidyl-tRNA- binding) sites on the ribosome is quite plausible and would explain how the tRNAs are conformationally stabilized to bring about precise codon reading and peptidyl transfer. The role of EF-G during the translocation would be to dissociate the peptidyl-tRNA and deacyl- ated tRNA from the A- and P- sites, respectively zg, and the final accommo- dation of aa-tRNA and peptidyl.tRNA

into the A- and P-sites would be fulfilled by two molecules of EF-Tu.

The provocative hypothesis outlined above demonstrates that, 28 years after the discovery of EF-Tu 3°, we are just beginning to understand the structure, function and mechanism of this key protein required for protein biosyn- thesis. The fact that EF-Tu also provides a molecular model for other members of the large family of regulatory GTPases makes its study even more challenging.

Acknowledgements The research laboratory in Bayreuth

was supported by the Deutsche Forschungsgemeinschaft, Slq3 213, D5 and Fonds der Chemischen industrie. I thank R. Hilgenfeld and J. Nyborg for cooperation on the study of the EF- Tu-GppNHp structure, my co-workers for the many contributions to which ! refer in this article, P. Hoffmiiller and S. Ribeiro for preparation of the drawings, and M. Daniel for help with preparation of the manuscript.

References 1 Kaziro, Y. (1978) Biochim. Biophys. Acta 505,

95-127 2 Yarus, M. (1992) Trends Biochem. Sci. 17,

130-133 3 Yarus, M. (1992) Trends Biochem. Sci. 17,

171-174 4 Nierhaus, K. H. (1993) Mol. Microbiol. 9,

661-669 5 Parmegglanl, A, and Swart, G, W. M. (1985)

Annu, Rev. Microbic/, 39, 557-577 6 Bourne, H. R., Sanders, D. A. and McCormick, F.

(1990) Nature 348, 125-132 7 MIIburn M. V. eta/. (1990) Science 247,

939-945 8 Pat, E. F. eta/ . (1989) Nature 314, 209-214

TIBS 19 - JUNE 1 9 9 4

9 Kjeldgaard, M. and Nyborg, J. (1992) J. 114o1. Biol. 223, 721-742

10 Berchtold, H. eta/. (1993) Nature 365,126-132 11 Kjeldgaard, M., Nissen, P., Thirup, S. and

Nyborg, J. (1993) Structure 1, 35-50 12 Noel, J. P., Hamm, H. E. and Sigler, P. B. (1993)

Nature 366, 654-663 13 Marshall, M. S. (1993) Trends Biochem. Sci.

18, 250-254 14 Wittinghoffer, A., Frank, R. and Leberman, R.

(1980) Eur. J. Biochem. 108, 423-431 15 Ott, G., Jonak, J., Abrahams, P. and Sprinzl, M.

(1990) Nucleic Acids Res. 18, 437-441 16 Peter, M. E., Schirmer, N. K., Reiser, C. O. A.

and Sprinzl, M. (1990) Biochemistry 29, 2876-2884

17 Hwang, Y-W., Sanchez, A. and Miller, L. (1989) in The Guanine Nuc/eotide Binding Proteins (NATO-ASI Series, Vol. 165) (Bosch, L., Kraal, B. and Parmeggiani, A., eds), pp. 77-85, Plenum

18 Bourne, H. R., Sanders, D. A. and McCormick, F. (1991) Nature 349, 117-127

19 Ott, G. and Sprinzl, M. (1992) in Structural Tools for the Analysis of Protein-Nucleic Acid Complexes (Lilley, D. M. J., Heumann, H. and Suck, D., eds), pp. 323-342, Birkh~user

20 Heerschap, A., Waiters, J. A. L. I., Mellema, J-R. and Hilbers, C. W. (1986) Biochemistry25, 2707-2713

21 F~)rster, C. et al. (1994) Biochimie 75, 1159-1166

22 F~rster, C., Limmer, S., Zeidler, W. and Sprinzl, M. (1994) Prec. Natl Acad. Sci. USA 91, 4254-4257

23 Joshi, R. L. et al. (1984) Nucleic Acids Res. 12, 7467-7478

24 Schwartzbach, C. J. and Spremulli, L. L. (1991) J. Biol. Chem. 266, 16324-16330

25 Schirmer, N. K., Reiser, C. O. A. and Sprinzl, M. (1991) Eur. J. Biochem. 200, 295-300

26 Hwang, Y. W., Carter, M. and Miller, D. L. (1992) J. Biol. Chem. 267, 22198-22205

27 WeIjland, A. and Parme~ianl, A. (1993) Science 259, 1311-1314

28 Ehrenberg, M,, Rojas, A, M,, Weiser, J. and Kurland, C. G. (1990) J. Me/. Biol. 211, 739-750

29 Holschuh, K., Bonln, J. and Gassen, H. G. (1980) Biochemistry 19, 5857-5864

30 Lucas-Lenard, J. and Lipmann, F. (1966) Prec. Natl Acacl. Sci. USA 55, 1562-1566

E R R A T U M

In the May issue of T/BS, we published the review 'Poly(ADP-ribose) polymerase: a molecular nick-sensor', by Gilbert de Murcia and Josiane M~nissier de Murcia (TIBS 19, 172-176). Unfortunately, Figures i and 3 contained several misleading errors. We apologise to the authors of the article and to our readers for any confusion caused. The correct figures are printed below.

F i g u r e I

Ade Ade I I

I ' "R R - - R R - - O H I I I I

P - p P - p

pNAZ)

~ _ c d O r Ade-] / Ade F Ade--1 Ade Narn/de

R - ~ R g - ~ R R3~---R R - O H ~ R ~l--OH ~ ' o ~ ° ' j '~'' , 2 j , ,,j i I i

IL/IpJ'i" = / i I / I I / I I ii I I -P-ix P-P LP-PJY P-P I P~'P

@ ; ~ Poly(ADP-ribose)glycohydrolase I I I t I I I I I I VE[ I I

- - AD P-ribosyl protein lyase DNA breaks

F i g u r e 3

250