Conversion of tRNA precursors to their mature forms requires
the action of both endo- and exoribonucleases. Although studies
over many years identified the endoribonuclease, RNase P, and
several exoribonucleases as the enzymes responsible for generating
the mature 5′ and 3′ termini, respectively, of
Escherichia coli tRNAs, relatively little is known
about how tRNAs are separated from long multimeric or multifunction
transcripts, or from long leader and trailer sequences. To examine
this question, the tRNA products that accumulate in mutant strains
devoid of multiple exoribonucleases plus one or several
endoribonucleases were analyzed by northern analysis. We find
that the multifunction tyrT transcript, which contains
two tRNA1Tyr sequences separated by a 209-nt
spacer region plus a downstream mRNA, is cleaved at three sites in
the spacer region by the endoribonuclease, RNase E. When both RNase E
and RNase P are absent, a product containing both tRNAs accumulates.
Two multimeric tRNA transcripts, those for tRNA Arg-His-Leu-Pro and
tRNA Gly-Cys-Leu also require RNase E for maturation. For the former
transcript, products with long 3′ extensions on tRNAArg,
tRNAHis, and tRNAPro, as well as the primary
transcript, accumulate in the absence of RNase E. For the latter
transcript, RNase E cleaves downstream of each tRNA. Little processing
of either multimeric transcript occurs in the absence of both RNase E
and RNase P. These data indicate that RNase E is a major contributor
to the initial processing of E. coli tRNA transcripts, providing
substrates for final maturation by RNase P and the 3′
exoribonucleases. Based on this new information, a detailed
model for tRNA maturation is proposed.