| Summary: | We have shown that the synthesis of active su+III tRNATyr from a phi80psu+III DNA template requires the action of four distinct enzymatic activities. The first of these, DNA-dependent RNA polymerase, catalyzes the formation of a large molecular weight transcript, initiating synthesis at a specific site 41 nucleotides proximal to the 5' end of the su+III tRNATyr structural gene and continuing at least 100 nucleotides beyond the 3' terminus of the su+III tRNATyr sequence. The second required component, designated Fraction V, allows purified DNA-DEPENDENT RNA polymerase to function in tRNA synthesis. We have shown that this fraction contains an endonuclease that together with DNA-dependent RNA polymerase is responsible for the synthesis of su+III tRNATyr "precursor". Thus, su+III tRNATyr precursor is not itself the primary transcription product of the su+III tRNATyr gene, but rather, it arises as a result of post-transcriptional cleavage of a much larger transcript by the action of the nuclease present in Fraction V. The third enzymatic activity required for synthesis of active su+III tRNATyr is a ribonuclease (RNase P III) that specifically catalyzes the removal of the 3' extra nucleotides from the su+III tRNATyr precursor. The fourth activity required for synthesis of tRNA is a previously identified endonuclease, RNase P, that specifically catalyzes the removal of the 5' extra nucleotides from tRNA precursors. The properties of RNase P purified according to the procedure developed in this laboratory have been compared with those of the enzyme purified from ribosomes according to the procedure described by Robertson et al. (Robertson, H.D., Altman, S., and Smith, F.D. (1972) J.Biol. Chem. 247, 5243-5251.).
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