Identification of precursor molecu. from Bombyx mori

Short-labeled 4.5S RNA molecules isolated from the posterior silk gland of Bombyx mori can be separated by two-dimensional polyacrylamide gel electrophoresis into many discrete species, some of which are radiochemically pure by the criteria of RNA fingerprinting. One region of the gel contains two precursor RNAs, one to each of the known alanine transfer RNAs. Each precursor tRNAAla molecule contains all of the in- ternal oligonucleotides present in the corresponding tRNAAla species plus new 5'- and 3'-terminal sequences. Precursor mol- ecules to tRNAGly (which differ from each other in size) are contained in two other gel regions, and a fourth region contains a precursor to tRNA2'Y. Both of the transcription initiator purine tetraphosphate nucleosides are present in unfractionated tRNA precursor mixtures, with pppA- predominating over pppG-. Minor nucleotides are also present in B. mori tRNA precursors. No polycistronic tRNA gene transcripts were observed. tRNA molecules are synthesized first as larger precursor species that are subsequently shortened and modified enzymatically to yield functional tRNAs. Although RNAs with these properties were first observed in mammalian cells (1, 2) and insect cells (3), only studies of the synthesis of tRNAs encoded by bacterial and bacteriophage genes have given the details of tRNA pre- cursor structures and an understanding of the highly specific nucleases involved in removal of the extra nucleotide sequences