Resumen de Study of a master regulator of recombination in Mycoplasma genitalium /
Mycoplasma genitalium is a small human pathogen that causes non-gonococcal urethritis, cervicitis and several pelvic inflammatory diseases. It bears one of the smallest genomes in an autonomous living organism and, for this reason, it has become a bacterial model for a minimal cell. Moreover, this small pathogen has a singular mechanism for antigenic variation that relies on the homologous recombination between the main adhesins and genomic repeats scattered around the chromosome. Despite that M. genitalium was one of the first microorganisms to be fully sequenced, very little is known of its reduced genetic circuitry and gene regulation. Besides, the regulatory mechanisms controlling the antigenic variation process are obscure notwithstanding it is essential for immune evasion and persistence. In this doctoral thesis, we have studied the role of the MG428 protein (sig20), which is an alternative sigma factor of this small bacterium. MG428 recognizes a novel promoter sequence and activates transcription of a small regulon composed by genes coding for key recombination enzymes, genes coding for hypothetical proteins and also non-coding regions with unknown function. Moreover, single cell analyses confirmed that activation via sig20 only takes place in a small subset of the population at the same time and showed a certain spatial association between sig20-activated cells. The activity of this alternative sigma factor is crucial for the recombination capacity displayed by M. genitalium and the generation of genetic variants of the main antigens in this bacterium. In this thesis, we have also focused on the regulation of sig20 activity by two uncharacterized proteins, named RrlA and RrlB (recombination regulatory loci). These regulators are under the transcriptional control of MG428 and stabilize sig20 protein, allowing the progression of a feed-forward loop required for the activation of the 20 regulon. Mutants of either rrlA or rrlB have similar recombination deficiencies as observed in an MG_428 null mutant. Finally, we have observed an unprecedented horizontal transfer of DNA between two different M. genitalium strains. DNA transfer occurs from a donor cell overexpressing sig20 to a recipient cell by means of homologous recombination.
Overall, in this doctoral thesis we have shown a novel transcriptional regulator of recombination in M. genitalium that promotes antigenic variation through the activation of transcription of a unique regulon. Sig20 can also activate an unorthodox horizontal gene transfer system that may play a key role in the evolution and adaptation of these small pathogens.
