The T4 endoribonuclease RegB is involved in the inactivation of the phage early messengers. It cuts specifically in the middle of GGAG sequences found in early messenger intergenic regions but not GGAG sequences located in coding sequences or in late messengers. In vitro RegB activity is very low but is enhanced by a factor up to 100 by the ribosomal protein S1. In the absence of clear sequence motif distinguishing substrate and non-substrate GGAG-containing RNAs, we postulated the existence of

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... a structural determinant. To test this hypothesis, we correlated the structure, probed by NMR spectroscopy, with the cleavage propensity of short RNA molecules derived from an artificial substrate. A kinetic analysis of the cleavage was performed in the presence and absence of S1. In the absence of S1, RegB efficiently hydrolyses substrates in which the last G of the GGAG motif is located in a short stem between two loops. Both strengthening and weakening of this structure strongly decrease the cleavage rate, indicating that this structure constitutes a positive cleavage determinant. Based on our results and those of others, we speculate that S1 favors the formation of the structure recognized by RegB and can thus be considered a "presentation protein."