In this work, we study the distribution of G + C content against chromosome size among 640 fully sequenced bacterial chromosomes. When %G + C is plotted versus increasing chromosome length, it is shown that the points denoting 640 bacterial chromosomes are widely scattered in the upper triangular area and the triangle is right-angled. The mean G + C content increases while the range or variance of G + C contents decreases with length. The net effect of these two trends is to exclude data from a triangular area that should represent long A + T-rich chromosomes. Thus, the shorter chromosomes have a wide, 20–70%, range of G + C contents while the longest chromosomes are severely restricted to a G + C content near 70%. To generate the distribution type of right-angled triangle, two conditions are requisite. Consequently, two possible related explanations are presented to interpret the relationship. One aims to give the reason why most large bacterial chromosomes have high G + C contents and the other tries to interpret why most A + T-rich bacterial chromosomes tend to be small in size. Both of the two interpretations are proposed from the selectionist viewpoint and the latter is an addition to the former one. According to the theories, the bacteria with large chromosomes and bacteria with A + T-rich chromosomes have different tactics to cope with ultraviolet radiation and other physical or chemical damage.
bacterial chromosome chromosome size G + C content triangular type obligate pathogen symbiont
A + T
adenine and thymine
G + C
guanine and cytosine
kilo base pairs
national center for biotechnology information
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We thank Professor Zi-Li You for useful discussion. We also thank the anonymous reviewers and Professor Pat Heslop-Harrison, the associate editor of Chromosome Research for valuable suggestions. The present study was supported by the National Natural Science Foundation of China (grant 60801058), the Committee of Science and Technology of Sichuan Province (grant 2008JY0053), and the Youth Research Foundation at UESTC (grant JX0769 and JX0643).
Supplementary Fig. 1The distribution of G+C content against chromosome length for bacterial chromosomes in 12 phylums, respectively. In the figure, each point corresponds to a bacterial chromosome. The red circles denote obligate pathogens or obligate symbionts. (DOC 663 KB)
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