On the control of transcription of bacteriophage Mu.

Abstract

The transcription pattern of bacteriophage Mu has been studied with the use of Mu-1 cts62, a thermo-inducible derivative of wild-type Mu. The rate of transcription at various times after induction was measured by pulse-labeling the RNA during synthesis and determining the fraction of Mu-specific RNA by hybridization with the separated strands of Mu-DNA. Transcription was found to take place predominantly from the heavy strand of Mu-DNA, as was found previously by Bade (1972). A study of the kinetics of this process revealed four phases. Initially after the induction the rate of transcription increases and reaches a maximum after four minutes. In the second phase during five minutes the rate falls down. During the third phase, up to 25 minutes after induction, the rate of transcription rises slowly, followed by a very rapid increase in the final phase, at the end of the lytic cycle. Phage Mu can be integrated in the host chromosome in two opposite orientations. The strand specificity, rate and time-course of transcription appeared not to be influenced by the orientation. The presence of chloramphenicol during the induction of the phage does not have an effect on the initial phase of transcription, but it prevents the decrease in the second phase. This suggests that in the early phase a Mu-specific protein is synthesized which acts as a negative regulator of trancription. In non-permissive strains, lysogenic for a phage with an amber mutation in gene A or B, the transcription during the first and the second phase is the same as with wild-type phage; in the third phase, however, there is much less transcription than with wild type phage, whereas in the final phase the increase of the transcription rate is completely absent.