Temporal control of message stability in the life cycle of double-stranded RNA bacteriophage phi8.

The cystoviruses have genomes of three double-stranded RNA segments. The genes of the L transcript are expressed early in infection, while those of M and S are expressed late. In all cystovirus groups but one, the quantity of the L transcript late in infection is lower than those of the other two because of transcriptional control. In bacteriophage Phi8 and its close relatives, transcription of L is not controlled; instead, the L transcript is turned over rapidly late in infection. The three messages are produced in approximately equal amounts early in infection, but the amount of L is less than 10% of the amounts of the others late in infection. The decay of the Phi8 L message depends upon the production of protein Hb, which is encoded in segment L. It also depends upon a target site within the H gene region. Phage mutants lacking either the Hb gene or the target region do not show the late control of L message quantity. In addition to having a role as a negative regulator, Hb functions to neutralize the activity of protein J, encoded by segment S, which causes the degradation of all viral transcripts.