by S.Palizban

different forms of bacterial chromosomes

in most of the bacteria:

single circular DNA moleculeHaploid

Exceptions:

•Bacteria with linear chromosomes

•Bacteria with more than one chromosome

Bacteria with more than one chromosome

bacteria have 2, 3 or 4 dissimilar chromosomesVibrio cholerae and Brucella melitensis have 2

dissimilar chromosomes

The existence of multiple chromosomes in bacteria has been known for some time. Yet the extent of functional solidarity between different chromosomes remains unknown.

Rhodobacter sphaeroides

•Multichromosomal •a kind of purple bacteria; • photosynthetic eubacterium •a rod-shaped, Gram-negative bacterium

The genome of R. sphaeroides . two chromosomes,

one of 3 Mb (CI) and one of 900 Kb (CII),

five naturally occurring plasmids.

Many genes are duplicated between the two chromosomes but appear to be differentially

regulated. Moreover, many of the  (ORFs) on CII seem to

code for proteins of unknown function.

When genes of unknown function on CII are disrupted, many types of auxotrophy result,

emphasizing that the CII is not merely a truncated version of CI

Vibrio cholerae;

the etiologic agent of the diarrheal disease, is a Gram-

negative bacterium . 

V. cholerae has two circular chromosomes ,

together totaling 4 million bp of DNA sequence and 3,885

predicted genes 

Chromosome 1; The larger first chromosome

contains the crucial genes for toxicity, regulation of toxicity, and important cellular functions, such as transcription and translation

The second chromosome

•Which is determined to be different from a plasmid or megaplasmid due to the inclusion of housekeeping and other essential genes in the genome, •including essential genes for metabolism, heat-shock proteins, and 16s rRNA genes

• Also relevant in determining if the replicon is a chromosome is whether it represents a significant percentage of the genome, and chromosome 2 is 40% by size of the entire genome.

•And, unlike plasmids, chromosomes are not self-transmissible.

However, the second chromosome may have once

been a megaplasmid because it contains some genes usually

found on plasmids.

The V. cholerae Genome Contains Two Replicons

Brucella sp.a small, Intracellular

Gram-negative coccobacillus,can causes brucellosis.

contains two unique and independent replicons

The availability of 10 different genomes consisting

of two chromosomes and

General features of the genomes:

All  genomes studied have two circular chromosomesof about 2,100 and 1,150 kb.

Both have similar G+C content,

Lactococcus lactis

is a Gram-positive bacterium used extensively in the production of buttermilk and cheese

In contrast to this general rule for bacteria, we found that Lactococcus lactis, is born with two complete non-replicating chromosomes. L. lactis therefore remain diploid throughout its entire life cycle.

.laboratory strains of Lactococcus lactis:

MG1363 and IL1403 Laboratory Strains

This strain thus fulfills the criterion of being diploid without overlapping chromosomal replication cycles.

we found that some of these strains were born with two complete nonreplicating chromosomes. We determined the cellular content of DNA by flow cytometry and by radioactive labeling of the DNA. These strains thus fulfill the criterion of being diploid.MG1363 and IL1403 are diploid in slow-growing cultures.

The diploid and haploid strains differed in :•their sensitivitytoward UV light, •in their cell size, •and in their D period, •the period between termination of DNA replication•and cell division.

Bacteria can carry plasmids and can be infected with viruses, each of which are capable of carrying copies of bacterial genes. Thus bacteria can be partially diploid, or merodiploid, for some genes.

Some prokaryotes have a linear chromosome;

Borrelia burgdorferi.

Borrelia burgdorferi;

the aetiological agent of Lyme disease, of spirochete

The chromosome of Borrelia behaved as a eukaryotic linear chromosome with a size of around 1,000 kb.

The genome also comprised several circular and linear plasmids which varied in size from 15 to 60 kb.

Bacteria Chromosome OrganizationAgrobacterium tumefaciens One linear and one circular

Bacillus subtilis Single and circularBacillus subtilis Single and linear

Borrelia burgdorferi Two circularBrucella abortus Two circular

Brucella melitensis Two circularBrucella ovis Two circular

Brucella suis biovar 1 Two circularBrucella suis biovar 2 Two circularBrucella suis biovar 4 Two circular

Escherichia coli Single and circularParacoccus denitrificans Three circularPseudomonas aeruginosa Single and circularRhodobacter sphaeroides Two circular

Streptomyces griseus LinearVibrio cholerae Two circularVibrio fluvialis Two circular

Vibrio parahaemolyticus Two circularVibrio vulnificus Two circular

Summary of Currently Known Bacterial Chromosome Organizations

Linear chromosomes are also found in other bacterial lineages,

including the pathogenic spirochaetes Borrelia afzelii, Borrelia burgdorferi and Borrelia garinii and several Streptomyces sppand the α-proteobacterium Agrobacterium tumefaciens.

A remarkable property of the 8.2-Mb chromosome of S. erythraea was that, contrary to the expectations and the earlier data (Reeves et al., 1998), it proved to be circular.

The chromosomes of its close relatives, Streptomyces coelicolor and Streptomyces avermitilis, are both linear, as is the chromosome of Rhodococcus sp. strain RHA1.

It is examined the properties of the Escherichia coli K-12 strains with a linear chromosome.

Chromosomes in eukaryotes are linear, whereas those of most, but not all, prokaryotes are circular. To explore the effects of possessing a linear genome on prokaryotic cells, we linearized the Escherichia coli genome using the lysogenic lambda-like phage N15. Linear genome E. coli were viable and their genome structure was stable. There were no appreciable differences between cells with linear or circular genomes in growth rates, cell and nucleoid morphologies, genome-wide gene expression (with a few exceptions), and DNA gyrase- and topoisomerase IV-dependent growth. However, under dif-defective conditions, only cells with a circular genome developed an abnormal phenotype. Microscopy indicated that the ends of the linear genome, but not the circular genome, were separated and located at each end of a new-born cell. When tos - the cis-element required for linearization - was inserted into different chromosomal sites, those strains with the genome termini that were more remote from dif showed greater growth deficiencies.

In fact linear chromosome even offered certain advantages: E. coli strains with circular chromosomes whose chromosomal recombination was affected by a mutation in the XerCD recombinase or by a deletion of the dif site, exhibited much slower growth than the same mutants carrying linearized chromosomes

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