We screened for mutant strains of Saccharomyces ces cerevisiae that are sensitive to overexpression of specific cyclins, and identified mutations in two genes that caused growth inhibition in response to mild overexpression of Clb3. One was the ANP1 gene, which encodes a glycosyltransferase previously identified by a similar strategy using Clb2 instead of Clb3. This paper describes the second strain of S. cerevisiae that is hypersensitive to Clb3 expression. The gene mutated in this strain was identified as PMR1, which encodes a Ca2+ ATPase located in the Golgi membrane. The protein product of pmr1-1 was truncated at residue 409 and thus lacked the C-terminal ATPase domain. The pmr1-1 strain was hypersensitive to over-expression of Clb3, but not Cln2, Clb5 or Clb2. The lethality due to Clb3 expression in pmr1-1 could be suppressed by adding Ca2+ ions to the medium. The pmr1-1 strain proved to be defective in glycosylation, and the defects in glycosylation were exacerbated by high levels of Clb3. On induction of Clb3 expression in the pmr1-1 strain, the cells arrested at anaphase with an elongated daughter bud. We discuss possible interpretations of this synthetic lethal phenotype.
