Fig. 3.

Diagrammatic representation of four independent pathways of tRNA processing. (A). Processing of the glyW cyst leuZ polycistronic operon. RNase E initiates processing by cleaving the polycistronic transcript to release pre-tRNAs (11). Processing at the 5′ termini is carried out by RNase P. Maturation of the 3′ termini is usually carried out by RNase T and/or RNase PH. If these two enzymes are not present, RNase D and/or RNase BN can complete the process. (B) Processing of the monocistronic leuX transcript (51). The Rho-independent transcription terminator is removed exonucleolytically by PNPase. In the absence of PNPase, a combination of RNase P and RNase II can digest the terminator. Subsequently, RNase P matures the 5′ terminus, while RNase T and RNase PH complete the process at the 3′ terminus. (C) Processing of the valV valW polycistronic operon (67). RNase P separates valV and valW pre-tRNAs by cleaving at their respective mature 5′ ends while PNPase and RNase II shorten the 3′ Rho-depended terminator. Subsequently, 3′ → 5′ exonucleases (RNase T/RNase PH/RNase D/RNase BN) matures the 3′ ends. (D). Processing of the monocistronic proK transcript (63). RNase E removes the Rho-independent transcription terminator to generate the mature 3′ terminus without the need of any of the 3′ → 5′ exonucleases. RNase P cleaves at the mature 5′ end. Figure is not drawn to scale.