Kinetics and regulation of transcription of bacteriophage Mu.

Mu transcription was analyzed by hybridization of [3H]uridine pulse-labeled RNA from heat-induced Mu lysogens to Mu DNA restriction fragments on nitrocellulose blots. Based on their time of appearance and dependence on Mu functions, we have defined three classes of transcripts: early, middle, and late. Replication-defective prophages containing A or B amber mutations or a deletion of the beta (right) end produced only early RNA derived from the left-most 8 to 10 kb of the Mu genome. A replication-proficient C amber mutant exhibited similar early transcription but at later times also produced middle transcripts from a region including C, which encodes the activator of late transcription. The C mutant did not produce late transcripts from the right-most 26 kb of the Mu genome encoding genes involved in phage morphogenesis and release. These results indicate that Mu DNA replication is required for efficient expression of middle RNA, which is itself required for expression of late transcripts. Amber mutations in essential genes other than A, B, and C had no significant effect on transcription except for polarity of one E mutation. Uninduced Mu c+ and Mu cts prophages produced very low levels of Mu-specific RNA derived from several regions including the c (immunity) gene and the region between genes B and C.