RNA polymerases in strict endosymbiont bacteria with extreme genome reduction show distinct erosions that might result in limited and differential promoter recognition.

Strict endosymbiont bacteria present high degree genome reduction, retain smaller proteins, and in some instances, lack complete functional domains compared to free-living counterparts. Until now, the mechanisms underlying these genetic reductions are not well understood. In this study, the conservation of RNA polymerases, the essential machinery for gene expression, is analyzed in endosymbiont bacteria with extreme genome reductions. We analyzed the RNA polymerase subunits to identify and define domains, subdomains, and specific amino acids involved in precise biological functions known in Escherichia coli. We also perform phylogenetic analysis and three-dimensional models over four lineages of endosymbiotic proteobacteria with the smallest genomes known to date: Candidatus Hodgkinia cicadicola, Candidatus Tremblaya phenacola, Candidatus Tremblaya Princeps, Candidatus Nasuia deltocephalinicola, and Candidatus Carsonella ruddii. We found that some Hodgkinia strains do not encode for the RNA polymerase α subunit. The rest encode genes for α, β, β', and σ subunits to form the RNA polymerase. However, 16% shorter, on average, respect their orthologous in E. coli. In the α subunit, the amino-terminal domain is the most conserved. Regarding the β and β' subunits, both the catalytic core and the assembly domains are the most conserved. However, they showed compensatory amino acid substitutions to adapt to changes in the σ subunit. Precisely, the most erosive diversity occurs within the σ subunit. We identified broad amino acid substitution even in those recognizing and binding to the -10-box promoter element. In an overall conceptual image, the RNA polymerase from Candidatus Nasuia conserved the highest similarity with Escherichia coli RNA polymerase and their σ70. It might be recognizing the two main promoter elements (-10 and -35) and the two promoter accessory elements (-10 extended and UP-element). In Candidatus Carsonella, the RNA polymerase could recognize all the promoter elements except the -10-box extended. In Candidatus Tremblaya and Hodgkinia, due to the α carboxyl-terminal domain absence, they might not recognize the UP-promoter element. We also identified the lack of the β flap-tip helix domain in most Hodgkinia's that suggests the inability to bind the -35-box promoter element.