Transcription initiation of Mu mom depends on methylation of the promoter region and a phage-coded transactivator.

The product of the bacteriophage Mu gene mom modifies adenine residues of DNA within the consensus sequence CGAGCNPy, providing protection against various restriction endonucleases (ref. 1 and D. Kamp, personal communication cited in ref. 2). The mom gene is only expressed during lytic development of the phage. It is known that mom is nonfunctional in Escherichia coli host mutants in a gene (dam) which itself encodes an adenine methylation system. We show here that the E. coli dam gene is essential for transcription initiation of the mom gene, and that this dependence on dam seems to lie in a short segment preceding the mom coding region, which also contains the mom promoter. The sequence of this segment reveals the presence of dam methylation sites (GATC), and suggests a model for the regulation of mom gene expression based on DNA secondary structure, which may explain why mom is only expressed during phage lytic development. We also show that expression of phage-coded proteins (A, B and C) is needed for transactivation of mom transcription.