McLean M J, Wolfe K H, Devine K M
Department of Genetics, University of Dublin, Trinity College, Dublin 2, Ireland.
J Mol Evol. 1998 Dec;47(6):691-6. doi: 10.1007/pl00006428.
Variation in GC content, GC skew and AT skew along genomic regions was examined at third codon positions in completely sequenced prokaryotes. Eight out of nine eubacteria studied show GC and AT skews that change sign at the origin of replication. The leading strand in DNA replication is G-T rich at codon position 3 in six eubacteria, but C-T rich in two Mycoplasma species. In M. genitalium the AT and GC skews are symmetrical around the origin and terminus of replication, whereas its GC content variation has been shown to have a centre of symmetry elsewhere in the genome. Borrelia burgdorferi and Treponema pallidum show extraordinary extents of base composition skew correlated with direction of DNA replication. Base composition skews measured at third codon positions probably reflect mutational biases, whereas those measured over all bases in a sequence (or at codon positions 1 and 2) can be strongly affected by protein considerations due to the tendency in some bacteria for genes to be transcribed in the same direction that they are replicated. Consequently in some species the direction of skew for total genomic DNA is opposite to that for codon position 3.
在已完成全基因组测序的原核生物中，研究了沿着基因组区域在第三密码子位置的GC含量、GC偏斜度和AT偏斜度的变化。在所研究的9种真细菌中，有8种显示GC和AT偏斜度在复制起点处改变符号。在6种真细菌中，DNA复制的前导链在密码子位置3富含G-T，但在两种支原体物种中富含C-T。在生殖支原体中，AT和GC偏斜度围绕复制起点和终点对称，而其GC含量变化已显示在基因组的其他位置具有对称中心。伯氏疏螺旋体和梅毒螺旋体显示出与DNA复制方向相关的碱基组成偏斜的异常程度。在第三密码子位置测量的碱基组成偏斜度可能反映了突变偏好，而在序列中所有碱基上测量的偏斜度（或在密码子位置1和2处）可能会受到蛋白质因素的强烈影响，这是因为在某些细菌中，基因倾向于以与复制相同的方向转录。因此，在某些物种中，全基因组DNA的偏斜方向与密码子位置3的偏斜方向相反。
