37-S-PRECURSOR TO RIBOSOMAL-RNA IS THE PRIMARY TRANSCRIPT OF RIBOSOMAL-RNA GENES IN SACCHAROMYCES-CEREVISIAE

The primary precursor to ribosomal RNA, identified by pulse-chase and fingerprint analyses, was isolated from lysates of an osmotic-sensitive Saccharomyces cerevisiae mutant. This precursor rRNA molecule has an electrophoretic mobility oorresponding to 37 S RNA and an estimated molecular weight of about 2.8 × 106. The 5′-terminal nucleotides of the 37 S rRNA precursor, labelled in vivo with [32P]phosphate, were analysed by several techniques, including hydrolysis with alkali and specific enzymes and analysis of the products by DEAE-Sephadex and Dowex 1 column, and PEI-cellulose thin-layer, chromatography. The 37 S rRNA precursor molecules are shown to bear 5′-terminal triphosphates (and diphosphates) of guanosine and adenosine. These results identify the 37 S rRNA precursor molecules as the primary transcripts of ribosomal RNA genes in S. cerevisiae. They suggest also that processing of the primary rRNA precursor cannot Start normally before transcription of its 25 S rRNA segment is completed. © 1979.