DNA methylation ratchet governs progression through a bacterial cell cycle

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 104; no. 43; pp. 17111 - 17116
• Main Authors: Collier, Justine, McAdams, Harley H, Shapiro, Lucy
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 23.10.2007; National Acad Sciences
• Subjects: B lymphocytes; Bacteria; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Base Sequence; Biological Sciences; Caulobacter; Caulobacter - cytology; Caulobacter - enzymology; Cell Cycle; Cell cycle proteins; Cell division; Chromosomes; Deoxyribonucleic acid; DNA; DNA Methylation; DNA replication; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; Gene expression; Genes; Methylation; Molecular Sequence Data; Promoter Regions, Genetic - genetics; Protein synthesis; Site-Specific DNA-Methyltransferase (Adenine-Specific) - metabolism; Time Factors; Transcription, Genetic
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0708112104

The Caulobacter cell cycle is driven by a cascade of transient regulators, starting with the expression of DnaA in G₁ and ending with the expression of the essential CcrM DNA methyltransferase at the completion of DNA replication. The timing of DnaA accumulation was found to be regulated by the methylation state of the dnaA promoter, which in turn depends on the chromosomal position of dnaA near the origin of replication and restriction of CcrM synthesis to the end of the cell cycle. The dnaA gene is preferentially transcribed from a fully methylated promoter. DnaA initiates DNA replication and activates the transcription of the next cell-cycle regulator, GcrA. With the passage of the replication fork, the dnaA promoter becomes hemimethylated, and DnaA accumulation drops. GcrA then activates the transcription of the next cell-cycle regulator, CtrA, once the replication fork passes through the ctrA P1 promoter, generating two hemimethylated copies of ctrA. The ctrA gene is preferentially transcribed from a hemimethylated promoter. CtrA then activates the transcription of ccrM, to bring the newly replicated chromosome to the fully methylated state, promoting dnaA transcription and the start of a new cell cycle. We show that the cell-cycle timing of CcrM is critical for Caulobacter fitness. The sequential changes in the chromosomal methylation state serve to couple the progression of DNA replication to cell-cycle events regulated by the master transcriptional regulatory cascade, thus providing a ratchet mechanism for robust cell-cycle control.