Coupling of cell growth, genome replication and segregation and cell division is crucial for all living organisms, yet cell progression control is largely underexplored in archaea. In this work, we characterize a ribbon-helix-helix transcriptional regulator (gp21) of rudivirus SIRV2, which infects the hyperthermophilic crenarchaeon S. islandicus LAL14/1. This protein belongs to a clade of highly conserved proteins of the Sulfolobales with homologs in several families of crenarchaeal viruses. Overexpression of gp21 and other members of the clade including the host homolog SiL_0190, results in impairment of cell division, evidenced by growth retardation, cell enlargement and an increase in DNA content. Additionally, gp21 can either repress or induce the transcription of several genes involved in cell division, DNA replication and cellular metabolism. Both gp21 and its cellular homolog SiL_0190 bind to the motif AGTATTA present in the promoter of their target genes, which correspond to the regulatory motif previously identified in a subset of cyclic genes in S. acidocaldarius ccr-2 (Cell Cycle Regulon 2) box. We therefore rename this clade of proteins Ccr-2 and their binding motif as ccr-2 box. Our results suggest that Ccr-2 is a master regulator of the cell cycle that silences genes involved in cell division and drives progression to the S-phase in Sulfolobales, a function exploited by all known viruses to facilitate viral propagation.

Cell cycle regulation is crucial for all living organisms and is often targeted by viruses to facilitate their own propagation, yet cell cycle progression control is largely underexplored in archaea. In this work, we reveal a cell cycle regulator (aCcr1) carrying a ribbon-helix-helix (RHH) domain and ubiquitous in the Thermoproteota of the order Sulfolobales and their viruses. Overexpression of several aCcr1 members including gp21 of rudivirus SIRV2 and its host homolog SiL_0190 of Saccharolobus islandicus LAL14/1 results in impairment of cell division, evidenced by growth retardation, cell enlargement and an increase in cellular DNA content. Additionally, both gp21 and SiL_0190 can bind to the motif AGTATTA conserved in the promoter of several genes involved in cell division, DNA replication and cellular metabolism thereby repressing or inducing their transcription. Our results suggest that aCcr1 silences cell division and drives progression to the S-phase in Sulfolobales, a function exploited by viruses to facilitate viral propagation.