Housekeeping genes essential for pantothenate biosynthesis are plasmid-encoded in Rhizobium etli and Rhizobium leguminosarum

Abstract

<p>Abstract</p> <p>Background</p> <p>A traditional concept in bacterial genetics states that housekeeping genes, those involved in basic metabolic functions needed for maintenance of the cell, are encoded in the chromosome, whereas genes required for dealing with challenging environmental conditions are located in plasmids. Exceptions to this rule have emerged from genomic sequence data of bacteria with multipartite genomes. The genome sequence of <it>R. etli </it>CFN42 predicts the presence of <it>panC </it>and <it>panB </it>genes clustered together on the 642 kb plasmid p42f and a second copy of <it>panB </it>on plasmid p42e. They encode putative pantothenate biosynthesis enzymes (pantoate-β-alanine ligase and 3-methyl-2-oxobutanoate hydroxymethyltransferase, respectively). Due to their ubiquitous distribution and relevance in the central metabolism of the cell, these genes are considered part of the core genome; thus, their occurrence in a plasmid is noteworthy. In this study we investigate the contribution of these genes to pantothenate biosynthesis, examine whether their presence in plasmids is a prevalent characteristic of the <it>Rhizobiales </it>with multipartite genomes, and assess the possibility that the <it>panCB </it>genes may have reached plasmids by horizontal gene transfer.</p> <p>Results</p> <p>Analysis of mutants confirmed that the <it>panC </it>and <it>panB </it>genes located on plasmid p42f are indispensable for the synthesis of pantothenate. A screening of the location of <it>panCB </it>genes among members of the <it>Rhizobiales </it>showed that only <it>R. etli </it>and <it>R. leguminosarum </it>strains carry <it>panCB </it>genes in plasmids. The <it>panCB </it>phylogeny attested a common origin for chromosomal and plasmid-borne <it>panCB </it>sequences, suggesting that the <it>R. etli </it>and <it>R. leguminosarum panCB </it>genes are orthologs rather than xenologs. The <it>panCB </it>genes could not totally restore the ability of a strain cured of plasmid p42f to grow in minimal medium.</p> <p>Conclusions</p> <p>This study shows experimental evidence that core <it>panCB </it>genes located in plasmids of <it>R. etli </it>and <it>R. leguminosarum </it>are indispensable for the synthesis of pantothenate. The unusual presence of <it>panCB </it>genes in plasmids of <it>Rhizobiales </it>may be due to an intragenomic transfer from chromosome to plasmid. Plasmid p42f encodes other functions required for growth in minimal medium. Our results support the hypothesis of cooperation among different replicons for basic cellular functions in multipartite rhizobia genomes.</p