show Abstracthide AbstractCell cycle regulation is of paramount importance for all forms of life. Here we report that a conserved and essential cell cycle-specific transcription factor (designated as aCcr1) and its viral homologs control cell division in Sulfolobales. We show that the transcription level of accr1 reaches peak during active cell division (D-phase) subsequent to the expression of CdvA, an archaea-specific cell division protein. Cells over-expressing the 58-aa-long RHH (ribbon-helix-helix) family cellular transcription factor as well as the homologs encoded by large spindle-shaped viruses Acidianus two-tailed virus (ATV) and Sulfolobus monocaudavirus 3 (SMV3) display significant growth retardation and cell division failure, manifested as enlarged cells with multiple chromosomes. aCcr1 over-expression results in downregulation of 17 genes (>4-folds) including cdvA. A conserved motif, aCcr1-box, located between the TATA-binding box and the translation initiation site in the promoters of 13 out of the 17 highly repressed genes, is critical for aCcr1 binding. The aCcr1-box is present in the promoters of cdvA genes across Sulfolobales, suggesting that aCcr1-mediated cdvA repression is an evolutionarily conserved mechanism by which archaeal cells dictate cytokinesis progression, whereas their viruses take advantage of this mechanism to manipulate the host cell cycle. Overall design: S. islandicus REY15A cells were synchronized as previously described with slight modifications. Briefly, cells were first grown aerobically at 75°C with shaking (145 rpm) in 30 ml of STV medium. When the OD600 reached 0.6-0.8, the cells were transferred into 300 ml STV medium with an initial estimated OD600 of 0.05. When the OD600 reached 0.15-0.2, acetic acid was added at a final concentration of 6 mM and the cells were blocked at G2 phase of the cell cycle after 6h treatment.6 mM acetic acid can synchronize S. islandicus REY15A in the G2 phase. Then, the cells were collected by centrifugation at 3,000 g for 10 min at room temperature to remove the acetic acid and washed twice with 0.7% (w/v) sucrose. Finally, the cells were resuspended into 300 ml of pre-warmed STV medium and cultivated as above for RNA-seq analysis.