DNA methyltransferase-dependent transcription of the phage Mu mom gene.

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Abstract
The phage Mu mom gene controls an unusual DNA modification. Expression of the mom function requires an active host (dam+) [Escherichia coli] 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. Transcription of the mom gene in dam+ and dam- cells was compared. 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 RNA blotted onto nitrocellulose filters (after fractionation by agarose gel electrophoresis). Mom-specific RNA was readily detected in dam+ lysogenic cells, but only after induction of the Mu prophage. The level of mom RNA was decreased at least to 1/20 th in induced dam- Mu lysogens. 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. The host (dam+) DNA adenine methylase activity was 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.