The mom gene of bacteriophage Mu: a unique regulatory scheme to control a lethal function

Abstract

The mom gene of bacteriophage Mu encodes a DNA modification function which converts adenine to acetamido adenine in a sequence-specific manner. The mom gene itself is subject to a complex regulation: gene expression requires methylation by the Escherichia coli Dam methylase of specific sites upstream of the mom promoter and transactivation of the promoter by a Mu gene product. The requirement for transactivation can be overcome when mom is transcribed from foreign promoters. When cloned into various sites in pBR322, the mom gene is always found in an orientation where transcription from vector promoters is excluded. The productive orientation is lethal to the cell. This effect is mediated by the concerted action of the mom gene product and the product of gene com (control of mom, previously termed ORF-x) whose coding region overlaps the 5'-coding region of the mom gene. When mom is expressed from its own promoter, internal deletions in com completely abolish expression of the mom gene. Fragments lacking the 5' end of com can be cloned downstream of constitutive plasmid promoters. The com gene product itself is not lethal to the cell. The region encoding mom has been cloned in p L expression vectors. The mom gene product, a peptide of 27 kDal, has been visualized on gels. Efficient expression of Mom from p L requires gene com. A fusion between MS-2 polymerase and com has been generated. The fusion product is made in large amounts, whereas the mom gene product is not overproduced although the gene is present on the same transcriptional unit. We show that the defect associated with com deletions can be complemented in trans, and propose that com acts as a positive regulator on a post-transcriptional level.