G. PANIS¹, M. DELABY¹, C. FUMEAUX¹, L. THERAULAZ¹, P.H. VIOLLIER<sup>1

1</sup>Department of Microbiology & Molecular Medicine, Faculty of Medicine, University of Geneva, Switzerland.

Chemical modifications impart secondary regulatory information on primary DNA sequence at the post-replicative level to regulate fundamental processes in human health and disease. These so-called epigenetic modifications are also present in bacterial genomes and introduced by sequence-specific DNA methyltransferases and control many DNA transactions such as gene transcription, DNA replication and repair and virulence functions. Recently, we made an important leap forward towards understanding how an epigenetic regulatory pair, composed by the CcrM methyltransferase and the enigmatic transcriptional regulator GcrA, controls cell cycle transcription in the synchronizable Caulobacter crescentus model.

In Caulobacter, cell cycle progression is controlled by a cyclical genetic circuit comprising at least four transcriptional regulators. We found that deletion of the gene encoding in the GcrA regulator resulted in (ΔgcrA) cells that are viable, but slow-growing and elongated, with the latter mostly due to an insufficiency of the FtsN cell division protein. Furthermore, suppressor analysis showed that a second cell cycle regulator, the CcrM methyltransferase, also causes a strong (filamentation) cell division defect when inactivated in isolation, while the simultaneous gcrA/ccrM disruption ameliorated the cytokinetic and growth defect of ΔgcrA cells, rather than aggravating it. As no direct biochemical and regulatory relationship between GcrA and CcrM were known before, we discovered that GcrA preferentially targets promoters harboring CcrM-dependent N6-methyl-adenosine promoter marks. Within the alphaproteobacteria, gcrA and ccrM are consistently present or absent together, rather than either gene being present alone, suggesting that gcrA/ccrM constitutes an independent, dispensable genetic module.

In addition, we found that functional analogues of the GcrA/CcrM pair are carried by a cell cycle-specific bacteriophage ϕCbK infecting Caulobacter. We identified that ϕCbK infection prevents expression of the host GcrA/CcrM module. Interestingly, ϕCbK replace the host epigenetic pair by phage encoded variants to reprogram the host cell cycle transcriptional program.