The RegB endoribonuclease from bacteriophage T4 cleaves early mRNAs specifically in the middle of the sequence GGAG. We show here that RegB is required for the degradation of bulk T4 early mRNA. In the absence of RegB, the chemical half-life of early transcripts is increased nearly fourfold, whereas their functional half-life is increased twofold. RegB also regulates the translation of several prereplicative genes. The synthesis of several early proteins is down-regulated, probably as a consequence of RegB cleavages in the Shine-Dalgarno sequence of these genes. The synthesis of several other proteins is up-regulated, suggesting that processing by RegB might improve translation by changing the conformation of a transcript. In contrast, RegB does not affect the average half-life of middle and late mRNA. An analysis of the susceptibility to RegB of many GGAG motifs carried by these mRNA species showed that most middle and all late GGAG-carrying mRNAs escape RegB processing in spite of the fact that the enzyme is acting at least until ten minutes post-infection. The sensitivity or resistance to RegB observed during phage infection could be reproduced in uninfected Escherichia coli cells and in vitro. This shows that the GGAG-carrying RNAs that are uncut during T4 infection are not substrates, whatever the period of the T4 cycle when the transcripts are made.
Copyright 2000 Academic Press.