Abstract
The two promoters (P
1 and P
2) of the cluster of transfer RNA genes of bacteriophage T4 are situated at distances of 0.95 × 10
3 and 1.3 × 10
3 bases, respectively, from the first tRNA gene. Isolated
in vitro primary transcripts initiated at these promoters were incubated with highly purified RNase III. The resulting cleavage products were mapped with the use of deletions in the T4 tRNA region. We found three primary RNase III cleavage sites in the leader region of the transcripts at positions about 40 (site 1), 150 (site 2) and 950 (site 3) nucleotides downstream from the promoter P
1. Analysis of the kinetics of transcript cleavage indicated that cleavage at sites 1 and 2 occurs simultaneously, presumably through a single act of interaction between RNase III and a double-stranded RNA stem. Cleavage at site 3 just before the first tRNA gene separates the clustered tRNA sequences from the leader part of the transcripts. This cleavage appears to be a major step in the T4 tRNA maturation pathway.