DNA methyltransferase-dependent transcription of the phage Mu mom gene

The phage Mu mom gene controls an unusual DNA modification. Expression of the mom function requires an active host (dam+) DNA adenine methylase [S-adenosyl-L-methionine:DNA (6-aminopurine)-methyltransferase]; in dam- hosts, Mu development is normal except that the viral DNA does not undergo the mom modification. The present communication compares transcription of the mom gene in dam+ versus dam- cells. 32P-labeled probes were prepared by nick-translation of a purified mom gene-containing restriction fragment and of virion DNA, respectively. These probes were hybridized with various RNAs blotted onto nitrocellulose filters (after fractionation by agarose gel electrophoresis). The salient findings are: (i) mom-specific RNA was readily detected in dam+ lysogenic cells, but only after induction of the Mu prophage; (ii) the level of mom RNA was decreased at least to 1/20th in induced dam- Mu lysogens; and (iii) little difference, if any, was observed between dam+ and dam- cells with respect to total Mu transcripts produced after prophage induction. These results are in accord with the known pattern of mom gene expression and Mu development. They show that the host (dam+) DNA adenine methylase activity is required for transcription of the mom gene. This represents a unique example where a DNA methylase exerts a positive regulatory role in mRNA transcription; alternative mechanisms for this process will be discussed.