POSTTRANSCRIPTIONAL CONTROLS IN BACTERIOPHAGE-T4 - ROLES OF THE SEQUENCE-SPECIFIC ENDORIBONUCLEASE REGB

Gene regB of bacteriophage T4 encodes a sequence-specific endoribonucl ease which introduces cuts in early phage messenger RNAs. In most case s, cutting takes place in the middle of the tetranucleotide GGAG. Effi cient cleavages occur in the motifs located in intergenic regions, som e of them being Shine-Dalgarno sequences. When located in a coding seq uence, this tetranucleotide is poorly recognized or not at all. In thi s article, we have reviewed the properties of the RegB endoribonucleas e, with emphasis on its possible roles in T4 development. We show that the nuclease RegB plays at least two roles: (i) it inactivates a sub- class of early mRNA by cleaving Shine-Dalgarno sequences, and (ii) it is necessary for the degradation of early mRNAs, but not of middle and late mRNAs. Accordingly, we found that middle and late mRNAs avoid pr ocessing by RegB, probably for different reasons. Most of the middle m RNAs (mRNAs initiated at MotA-dependent promoters) do not contain the motif GGAG in their intergenic regions, whereas about one-third of the late genes have this motif as Shine-Dalgarno sequence. It is not yet known whether the RNase is inactivated early in the phage cycle, or wh ether it remains active but cannot recognize late mRNAs as substrates.