Toyocamycin inhibition of ribosomal ribonucleic acid processing in an osmotic-sensitive adenosine-utilizing Saccharomyces cerevisiae mutant.

An adenosine-utilizing mutant of Saccharomyces cerevisiae (SY 15 ado) is isolated after remutagenesis of an osmotic-sensitive strain, auxotrophic for adenine, with ethyl methanesulfonate. It is shown that the SY15ado mutant can be used to achieve experimental conditions under which cell growth and RNA Synthesis are directly dependent on exogenous adenosine. After starvation for adenosine, toyocamycin is incorporated into pre-rRNA chains of SY15ado cells replacing adenosine residues. The extent of this replacement depends on the concentration of added toyocamycin. Lower doses slow down processing of pre-rRNA into mature rRNA with an accumulation of 27 S and 20 S pre-rRNA. At higher concentrations toyocamycin blocks the last steps of pre-rRNA processing i.e. the conversions 27 S pre-rRNA leads to 25 S rRNA and 20 S pre-rRNA leads to 18 S rRNA. It appears that the main site of toyocamycin action is at the last steps of ribosome formation, while transcription and the early stages of pre-RNA processing are less affected.