Several λtof mutants were isolated which overproduced λ exonuclease and l-strand mRNA upon infection. These tof mutations also affected r-strand transcription of the x-P region and were grouped in two classes (I and II) on the basis of their modes of r-strand transcription. Like wild-type λ, class I mutants maintained a constant rate of r-strand transcription throughout phage growth. Class II mutants exhibited a very slow initial rate of r-strand transcription, but the rate then increased gradually and eventually surpassed that of tof+at late times after infection. Overproduction of r-strand mRNA by class II mutants was turned off by mixed-infection with wild-type λ. Revertants were isolated at a frequency of 10-5to 10-6. λ Exonuclease synthesis, and l-strand and r-strand transcription were simultaneously restored to wild type levels. Other lines of evidence were also presented, which indicate that the overproduction of l-strand as well as r-strand mRNAs are not due to a newly acquired promoter or an operator mutation. It is concluded that the tof gene product regulates not only l-strand but also r-strand expression at the transcriptional level. It works essentially negatively, but it may act positively on the x-Q operon at low concentration. Regulation of early gene expression in λ phage is discussed in the light of these observations. © 1975.
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