Santoyo Gustavo, Martínez-Salazar Jaime M, Rodríguez César, Romero David
Programa de Ingeniería Genómica, Centro de Ciencias Genómicas-Universidad Nacional Autónoma de México, Apartado Postal 565-A, 62210 Cuernavaca, Morelos, México.
J Bacteriol. 2005 Jun;187(12):4116-26. doi: 10.1128/JB.187.12.4116-4126.2005.
Gene conversion has been defined as the nonreciprocal transfer of information between homologous sequences. Despite its broad interest for genome evolution, the occurrence of this mechanism in bacteria has been difficult to ascertain due to the possible occurrence of multiple crossover events that would mimic gene conversion. In this work, we employ a novel system, based on cointegrate formation, to isolate gene conversion events associated with crossovers in the nitrogen-fixing bacterium Rhizobium etli. In this system, selection is applied only for cointegrate formation, with gene conversions being detected as unselected events. This minimizes the likelihood of multiple crossovers. To track the extent and architecture of gene conversions, evenly spaced nucleotide changes were made in one of the nitrogenase structural genes (nifH), introducing unique sites for different restriction endonucleases. Our results show that (i) crossover events were almost invariably accompanied by a gene conversion event occurring nearby; (ii) gene conversion events ranged in size from 150 bp to 800 bp; (iii) gene conversion events displayed a strong bias, favoring the preservation of incoming sequences; (iv) even small amounts of sequence divergence had a strong effect on recombination frequency; and (v) the MutS mismatch repair system plays an important role in determining the length of gene conversion segments. A detailed analysis of the architecture of the conversion events suggests that multiple crossovers are an unlikely alternative for their generation. Our results are better explained as the product of true gene conversions occurring under the double-strand break repair model for recombination.
基因转换被定义为同源序列之间信息的非相互转移。尽管它对基因组进化具有广泛的研究意义，但由于可能发生的多重交叉事件会模拟基因转换，所以在细菌中该机制的发生一直难以确定。在这项研究中，我们采用了一种基于共整合体形成的新型系统，来分离与固氮细菌根瘤菌中交叉相关的基因转换事件。在这个系统中，仅对共整合体形成进行选择，而基因转换则作为未被选择的事件被检测到。这将多重交叉的可能性降至最低。为了追踪基因转换的范围和结构，我们在其中一个固氮酶结构基因（nifH）中引入了均匀间隔的核苷酸变化，为不同的限制性内切酶创造了独特的位点。我们的结果表明：（i）交叉事件几乎总是伴随着附近发生的基因转换事件；（ii）基因转换事件的大小范围从150 bp到800 bp；（iii）基因转换事件表现出强烈的偏向性，有利于保留导入序列；（iv）即使是少量的序列差异也对重组频率有很大影响；（v）MutS错配修复系统在确定基因转换片段的长度方面起着重要作用。对转换事件结构的详细分析表明，多重交叉不太可能是其产生的另一种方式。我们的结果更好地解释为在双链断裂修复重组模型下发生的真正基因转换的产物。
