Transcription of bacteriophage Mu

Summary

It has previously been shown that the transcription of Mu is asymmetric and takes place on the heavy DNA strand (Bade, 1972; Wijffelman et al., 1974). The direction of transcription of Mu has now been determined by RNA-DNA hybridizations between purified Mu-RNA and the separated strands of λ-Mu hybrid phages. The direction of transcription is from the c-gene (immunity gene) end of the heavy strand to the β-end (immunity distal end) (Fig. 1). Thermo-inducible, defective Mu lysogens, in which the prophage is deleted from the β-end, have a normal early transcription pattern, but the increase of RNA at later times is absent. A defective lysogen, which contains only the immunity gene c and the genes A and B, still has an early transcription pattern similar to that of the wild-type. Therefore, we conclude that the early RNA is transcribed from that region of the Mu genome.

The early Mu-RNA synthesis is negatively regulated with a minimum rate of transcription at 9 minutes after induction. Before the onset of the late RNA synthesis, at about 22 minutes there is a rather long period in which the rate of Mu-RNA synthesis slowly increases. Using DNA strands of λ-Mu hybrids which contain only that part of the Mu-DNA on which the early RNA synthesis takes place, we have determined that during the first half in the intermediate phase only early genes are transcribed.

The amount of Mu-RNA synthesized by a Mu prophage carrying the X-mutation, which influences the excision of Mu, is greatly reduced. Negative regulation of early transcription occurs normally in this mutant.