Structure of yeast phenylalanine-t: segment within the region coding f,

Sixteen bacterial clones containing sequences complementary to yeast Phe-tRNA were isolated from a col- lection of hybrid plasmids containing BamHI restriction en- donuclease-generated yeast DNA fragments inserted in the plasmid vector pBR315. Ten of these clones contained hybrid plasmids with distinct BamHI fragments. The sequence of the Phe-tRNA structural genes and adjacent regions of three of these clones is reported here. In the region flanking the tRNA gene, the sequence of two of the cloned DNAs is similar; the sequence of the third varies considerably. All three of the tRNA genes are bordered by A,T-rich regions. In particular, near the region coding for the 3' end of the tRNA there is a long sequence of As in the coding strand. This is reminiscent of the region of ter- mination of transcription of the yeast 5S rRNA gene. The se- quences coding for the Phe-tRNA contain an additional segment of 18 or 19 base pairs (depending upon the clone) not predicted by the yeast Phe-tRNA sequence. These intervening segments are nearly identical in the three clones and are located within the structural gene, two base pairs from the nucleotides coding for the tRNA anticodon. It is axiomatic that knowledge of the organization and structure of eukaryotic genes will clarify some of the mysteries of their expression and regulation. In order to study the factors required for specific transcription of ribosomal genes, we have recently determined the nucleotide sequence of a substantial fraction of the 9000-base-pair yeast ribosomal DNA repeat which contains both the 35S ribosomal precursor (transcribed by RNA polymerase I) and the 5S rRNA genes (transcribed by RNA polymerase III) (1-3). Inspection of the flanking sequences of the 5S rRNA gene (which appears to be an unprocessed gene product) showed a striking A-rich sequence in the coding strand