Decay of mRNAs employing nonspecific endoribonucleases. Kushner S R J. Bacteriol. 2002;184:4658-4665 Decay of mRNAs employing nonspecific endoribonucleases. In this model, an enzyme such as RNase M would initiate mRNA decay near the 5′ terminus of the transcript. Following additional cleavages by it and perhaps another endoribonuclease, such as RNase I*, the breakdown products would be exonucleolytically degraded by a combination of PNPase, RNase II, and RNase R. Secondary structures could be removed by the action of an RNA helicase and/or the addition of poly(A) tails, which has been shown to promote the movement of PNPase through a stem-loop structure (27). RBS, ribosome binding site; PAP I, poly(A) polymerase I.