Gene Conversion Tracts Associated with Crossovers in Rhizobium etli

Abstract

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.

Gorgeous mosaic of mitochondrial genes created by horizontal transfer and gene conversion

“…3) (12, 13). Gene conversion can occur either continuously or discontinuously (28,29), the latter consistent with the highly chimeric nature of the loci described herein.…”

Transformation and isolation of allelic exchange mutants of Chlamydia psittaci using recombinant DNA introduced by electroporation

“…In 1 recombinant, we did not see transfer of the mutation located in between the 2 resistance mutations to the bacterial chromosome. Similar recombinants, displaying 2 tracts of continuous gene replacement with a marker or two between these tracts that did not show sequence replacement, have been previously observed in Rhizobium etli (24). It was speculated that rather than emerging from a rare quadruple crossover, these discontinuous sequence transfers more likely resulted from gene conversion.…”

The Extent of Migration of the Holliday Junction Is a Crucial Factor for Gene Conversion in Rhizobium etli

“…1E and F). Consistent with these predictions, inactivation of bacterial systems participating in the generation of the degraded segment, such as the RecBCD or AddAB system (1,3), or in charge of mismatch repair, such as MutS (1,39), provokes a reduction in the frequency and/or extent of gene conversion in bacteria. Oddly enough, the extent of migration of the HJs on gene conversion has been a poorly studied factor.…”

“…In bacteria, this nonreciprocal transfer of information usually entails sizable gene segments (400 to 600 bp are frequent) and occurs at frequencies higher than the mutation frequency (1,38,39). The mechanism of gene conversion has as a consequence the spread of sequence polymorphisms present in one of the two recombining homologs; these polymorphisms can be either maintained or eliminated in both sequences, thus giving rise to identity between homologs.…”

“…To gain insight into the mechanism of gene conversion in R. etli, we have studied the anatomies of tracts undergoing gene conversion in this organism (39). Our results revealed that (i) crossover events were almost invariably accompanied by a gene conversion event occurring nearby; (ii) gene conversion tract lengths ranged in size from 150 bp up to 800 bp; (iii) gene conversion events displayed a strong bias, favoring the preservation of incoming sequences; and (iv) the MutS mismatch repair system plays an important role in determining the length of gene conversion segments (39).…”