The Saccharomyces cerevisiae Hyperrecombination Mutant hpr1Δ Is Synthetically Lethal with Two Conditional Alleles of the Acetyl Coenzyme A Carboxylase Gene and Causes a Defect in Nuclear Export of Polyadenylated RNA

In a screen for mutants that display synthetic lethal interaction with hpr1Δ, a hyperrecombination mutant of Saccharomyces cerevisiae, we have isolated a novel cold-sensitive allele of the acetyl coenzyme A (CoA) carboxylase gene, acc1(cs), encoding the rate-limiting enzyme of fatty acid synthesis. The synthetic lethal phenotype of the acc1(cs) hpr1Δ double mutant was only partially complemented by exogenous fatty acids. hpr1Δ was also synthetically lethal with a previously isolated, temperature-sensitive allele of ACC1, mtr7 (mRNA transport), indicating that the lethality of the acc1(cs) hpr1Δ double mutant was not allele specific. The basis for the interaction between conditional acc1 alleles and hpr1Δ was investigated in more detail. In the hpr1Δ mutant background, acetyl-CoA carboxylase enzyme activity was reduced about 15-fold and steady-state levels of biotinylated Acc1p and ACC1 mRNA were reduced 2-fold. The reduced Acc1p activity in hpr1Δ cells, however, did not result in an altered lipid or fatty acid composition of the mutant membranes but rendered cells hypersensitive to soraphen A, an inhibitor of Acc1p. Similar to mtr7, hpr1Δ and acc1(cs) mutant cells displayed a defect in nuclear export of polyadenylated RNA. Oversized transcripts were detected in hpr1Δ, and rRNA processing was disturbed, but pre-mRNA splicing appeared wild type. Surprisingly, the transport defect of hpr1Δ and acc1(cs) mutant cells was accompanied by an altered ring-shaped structure of the nucleolus. These observations suggest that the basis for the synthetic lethal interaction between hpr1Δ and acc1 may lie in a functional overlap of the two mutations in nuclear poly(A)(+) RNA production and export that results in an altered structure of the nucleolus.