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Abstract
The bacteriophage Mu mom gene encodes a novel DNA modification that protects the viral
genome against a wide variety of restriction endonucleases. Expression of mom is subject
to a series of unusual regulatory controls. Transcription requires the action of a
phage-encoded protein, C, which binds (probably as a dimer) the mom promoter from
-33 to -52 (with respect to the transcription start site) in two adjacent DNA major
grooves on one face of the helix. No apparent direct interaction between C and the
host RNA polymerase (RNAP) is evident; however, C binding alters mom DNA conformation.
In the absence of C, RNAP binds the mom promoter at a site that results in transcription
in a direction away from the mom gene. The function of this transcription is unknown.
An additional layer of transcriptional regulation complexity is due to the fact that
the host Dam DNA-(N6-adenine)methyltransferase is required. Dam methylation of three
closely spaced upstream GATC sequences is necessary to prevent binding by the host
protein, OxyR, which acts as a repressor. Repression is not mediated by inhibition
of C binding, but rather through interference with C-mediated recruitment of RNAP
to the correct site. Translation of mom is regulated by the phage Com protein. Com
is only 62 amino acids long and contains a zinc finger-like structure (coordinated
by four cysteine residues) in the amino terminal domain. Com binds mom mRNA 5' to
the mom open reading frame, whose translation start signals are contained in a stem-loop
translation-inhibition-structure. Com binding to its target site (5' to and adjacent
to the translation-inhibition-structure) results in a stable change in RNA secondary
structure that exposes the translation start signals.