A cold-sensitive mutant of Saccharomyces cerevisiae defective in ribosome processing

Cold-sensitive mutants of Saccharomyces cerevisiae isolated by tritium suicide were screened for defects in ribosome biosynthesis. The biochemical defects of mutant dip-1 (defective in processing) were characterized; it is defective in ribosome biosynthesis at the level of production of the primary 35S transcript. At restrictive conditions mutant dip-1 accumulates abnormal rRNA in addition to wild-type rRNA. In the mutant the first observable transcription product was a 14SRNA species which had sequence homologies to 18S rDNA and was the major rRNA component of the 40S ribosomal subunit. In addition, the ribonucleoprotein particles of dip-1 harbored RNA molecules with homologies to yeast rDNA which comprises the spacer region between 18S and 25S rDNA cistrons. Possible causes for the defective production of rRNA and its assembly into subunits are discussed.