Chromosome Size

Chromosome sizes of sequenced bacteria range from 580 kb of Mycoplasma geni-talium (7) to 9105 kb of Bradyrhizobium japonicum (8). The distribution of the sizes of bacterial genomes is shown in Fig. 2. Ninety percent of the bacterial genomes sequenced are less than 5.5 Mb. There are clusters of genomes around 0.9-1.3, 1.7-2.5, 3.3-3.7, and 4.5-4.9 Mb. Variations in the size of bacterial genomes are expected because there is huge diversity among bacteria in morphology, metabolic capability, and the ability to survive and grow in various environments or hosts. For some bacterial species, multiple strains have been sequenced: Bacillus anthracis, Bacillus cereus, Brucella meliten-sis, Chlamydophila pneumoniae, Escherichia coli, Leptospira interrogans, Myco-

1995 1996 1997 199B 1999 2000 2001 2002 2003 2004

Year

Fig. 1. Number of completed genomes per year from 1995 to 2004. The numbers for each year were derived from two web sites: http://www.ncbi.nlm.nih.gov/genomes/MICROBES/ Complete.html and http://www.genomeonline.org.

Fig. 2. Size distribution of bacterial genomes. The sizes of the bacterial genomes were from two web sites: http://www.ncbi.nlm.nih.gov/genomes/MICROBES/Complete.html and http:// www.genomeonline.org.When multiple strains of the same species were sequenced the mean size of the different strains were used, provided the size variation was less than 10%. However, if the size variation exceeded 10%, they were entered as separate genomes.

Fig. 2. Size distribution of bacterial genomes. The sizes of the bacterial genomes were from two web sites: http://www.ncbi.nlm.nih.gov/genomes/MICROBES/Complete.html and http:// www.genomeonline.org.When multiple strains of the same species were sequenced the mean size of the different strains were used, provided the size variation was less than 10%. However, if the size variation exceeded 10%, they were entered as separate genomes.

bacterium tuberculosis, Neisseria meningitidis, Prochorococus marinus, Salmonella enterica Typhi, Shigella flexneri, Staphylococcus aureus, Streptococcus agalactiae, Streptococcus pyogenes, Tropheryma whipplei, Vibro vulnificus, Xylella fastidiosa, and Yersiniapestis (see http://www.genomeonline.org; http://www.ncbi.nlm.nih.gov/ genomes/MICROBES/Complete.html). With the exception of E. coli and P. marinus, different strains of the same species have a very similar genome size. E. coli A12-MG1655 is 4.6 Mb, whereas the largest strain sequenced, E. coli O157:H7 (Sakai) is 5.6 Mb, 18% larger. Two ecotypes (strains of the same species occupying a different niche) of P. marinus, MED4 and MIT9313, were recently sequenced. The genome of both strains consists of a single circular chromosome. The chromosome of MED4, a high-light-adapted strain, is 1658 kb and that of MIT9313, a low-light-adapted strain, is 2411 kb. That is, the genome of ecotype MED4 is only 68.8% the size of MIT9313 and yet, on the basis of their ribosomal DNA sequences, they are classified as the same species.

There are fewer archaeal genomes sequenced than bacterial genomes. Even though chromosomes of archaeal genomes can also differ greatly in size, ranging from the smallest at 500 kb for Nanoarchaeum equitans (9) to 5751 kb forMethanosarcina ace-tivorans (10), the sizes of archaeal genomes are not as varied as those of bacteria. Fourteen of the 20 archaeal species sequenced to date have a genome size between 1.5 and 2.3 Mb (see Fig. 3).

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