Cell cycle transition from S-phase to G1 in Caulobacter is mediated by ancestral virulence regulators

We report that ancestral zinc-finger-domain transcriptional regulators, previously reported to control virulence/symbiosis, implement a cell cycle (S?G1) transcriptional switch. To unravel how this G1-phase transcriptional program is reinstated during a primitive cell cycle, we first defined G1-specific promoters in the model bacterium Caulobacter crescentus by comparative ChIP-Seq analysis. We then exploited one such promoter as genetic proxy, to identify two conserved developmental regulator paralogs, MucR1/2, that constitute a quadripartite and homeostatic regulatory module directing the switch from S?G1-phase transcription. Surprisingly, MucR orthologs that regulate virulence and symbiosis gene transcription in Brucella, Agrobacterium or Sinorhizobium support the G1 transcriptional switch in Caulobacter. Pan-genomic ChIP-Seq analyses in Sinorhizobium and Caulobacter show that this module targets orthologous genes. Thus, this ancestral bacterial lineage from which eukaryotic organelles descended may coordinate virulence/symbiosis with other cell cycle functions using a primordial transcription factor fold that is now primarily found in the eukaryotic domain of life. Overall design: Examination of 5 transcripton factor binding in two different species