Abstract

Gene conversion is defined as the non-reciprocal transfer of genetic information from one site to a homologous, but not identical site of the genome. In prokaryotes, gene conversion can increase the variance of sequences, like in antigenic variation, but can also lead to a homogenization of sequences, like in the concerted evolution of multigene families. In contrast to these intramolecular mechanisms, the intermolecular gene conversion in polyploid prokaryotes, which leads to the equalization of the multiple genome copies, has hardly been studied. We have previously shown the intermolecular gene conversion in halophilic and methanogenic archaea is so efficient that it can be studied without selecting for conversion events. Here, we have established an approach to characterize unselected intermolecular gene conversion in Haloferax volcanii making use of two genes that encode enzymes involved in carotenoid biosynthesis. Heterozygous strains were generated by protoplast fusion, and gene conversion was quantified by phenotype analysis or/and PCR. It was verified that unselected gene conversion is extremely efficient and it was shown that gene conversion tracts are much longer than in antigenic variation or concerted evolution in bacteria. Two sites were nearly always co-converted when they were 600 bp apart, and more than 30% co-conversion even occurred when two sites were 5 kbp apart. The gene conversion frequency was independent from the extent of genome differences, and even a one nucleotide difference triggered conversion.

Highlights

  • Gene conversion is defined as the non-reciprocal transfer of information between two homologous, but not identical DNA sequences

  • In the present study we established the use of genes encoding enzymes involved in carotenoid biosynthesis for the characterization of unselected intermolecular gene conversion in the polyploid haloarchaeon H. volcanii

  • The species is ideally suited for the analysis of gene conversion, because (1) it can be cultivated in various media very and has a doubling time around 3 h, (2) the genome can be changed very due to an efficient genetic system, and (3) heterozygous cells with different genomes can be generated very using a protoplast fusion technique

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Summary

Introduction

Gene conversion is defined as the non-reciprocal transfer of information between two homologous, but not identical DNA sequences. It occurs in all three domains of life, archaea, bacteria, and eukaryotes. Gene conversion in eukaryotes has been reviewed repeatedly and will not be discussed here (Tang and Martin, 2007; Duret and Galtier, 2009; McCulloch et al, 2015; Korunes and Noor, 2017) It is a default event in meiosis and has a probability of 10−6 –10−4 per site in various eukaryotes (Korunes and Noor, 2017). Intermolecular Gene Conversion in Haloferax volcanii prokaryotes do not exhibit meiosis, and, gene conversion must have other functions in prokaryotes than enhancing genetic diversity during meiosis

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