RNA polymerases in strict endosymbiont bacteria with extreme genome reduction show distinct erosions that could result in limited and differential promoters’ recognition

Strict endosymbiont bacteria with high degree of genome reduction retain smaller proteins and, in certain cases, lack complete functional domains compared to their free-living counterparts. Until now, the mechanisms underlying these genetic reductions are not well understood. However, it is thought that, in order to compensate for gene reduction, somehow hosts take over those vital functions that endosymbionts cannot perform. In the present study, the conservation of RNA polymerases, the essential machinery for gene expression, is analysed in bacteria with extreme genome reductions. For this purpose, comparative genomics, phylogenetic analysis and three-dimensional models of RNA polymerase subunits were done over four lineages of endosymbiotic proteobacteria with the smallest genomes known to date. Amino acids under positive selection in the α subunit and loss of motifs in other subunits of RNA polymerase were observed. According to three-dimensional models, sites under positive selection might compensate the loss of motifs in α subunit. In addition, variations in the σ subunit were identified, some of them already studied in E. coli as a result of random mutagenesis. Amino acid changes in RNA polymerase suggest a possible modification in the binding specificity of the canonical −10 box (TATAAT) in some of these organisms. Furthermore, the β-flap helix domain is absent in some Hodgkinia strains, as observed in RNA pol II of Archaea, thus lacking the capacity to bind to the −35 box. Here, we propose several RNA polymerases models for endosymbiont bacteria with extremely reduced genomes. Evidence suggests that RNA polymerases of each endosymbiont bacteria follow a unique evolutionary path, without necessarily following the same path as a lineage, this is probably influenced by the intimate interactions sustained with other endosymbionts and its hosts.