Localization and regulation of bacteriophage Mu promoters.

Mu promoters active during the lytic cycle were located by isolating RNA at various times after induction of Mu prophages, radiolabeling it by capping in vitro, and hybridizing it to Mu DNA fragments on Southern blots. Signals were detected from four new promoters in addition to the previously characterized Pe (early), PcM (repressor), and Pmom (late) promoters. A major signal upstream of C was first observed at 12 min and intensified thereafter with RNA from cts and C amber but not replication-defective prophages; these characteristics indicate that this signal arises from a middle promoter, which we designate Pm. With 20- and 40-min RNA, four additional major signals originated in the C-lys, F-G-I, N-P, and com-mom regions. These signals were missing with RNA from C amber and replication-defective prophages and therefore reflected the activity of late promoters, one of which we presume was Pmom. Uninduced lysogens showed weak signals from five regions, one from the early regulatory region, three between genes B and lys, and one near the late genes K, L, and M. The first of these probably resulted from PcM activity; the others remain to be identified.