Whiteway M, Freedman R, Van Arsdell S, Szostak J W, Thorner J
Department of Molecular Biology, Massachusetts General Hospital, Boston 02114.
Mol Cell Biol. 1987 Oct;7(10):3713-22. doi: 10.1128/mcb.7.10.3713-3722.1987.
Mutations in the ARD1 gene prevent yeast cells from displaying G1-specific growth arrest in response to nitrogen deprivation and cause MATa haploids (but not MAT alpha haploids) to be mating defective. Analysis of cell type-specific gene expression by examination of RNA transcripts and measurement of beta-galactosidase activity from yeast gene-lacZ fusions demonstrated that the mating defect of MATa ard1 mutants was due to an inability to express genes required by MATa cells for the mating process. The lack of mating-specific gene expression in MATa cells was found to be due solely to derepression of the normally silent alpha information at the HML locus. The cryptic a information at the HMR locus was only very slightly derepressed in ard1 mutants, to a level insufficient to affect the mating efficiency of MAT alpha cells. The preferential elevation of expression from HML over HMR was also observed in ard1 mutants which contained the alternate arrangement of a information at HML and alpha information at HMR. Hence, the effect of the ard1 mutation was position specific (rather than information specific). Although the phenotype of ard1 mutants resembled that of cells with mutations in the SIR1 gene, both genetic and biochemical findings indicated that ARD1 control of HML expression was independent of the regulation imposed by SIR1 and the other SIR genes. These results suggest that the ARD1 gene encodes a protein product that acts, directly or indirectly, at the HML locus to repress its expression and, by analogy, may control expression of other genes involved in monitoring nutritional conditions.
ARD1基因的突变会阻止酵母细胞在氮缺乏时出现G1期特异性生长停滞，并导致MATa单倍体（而非MATα单倍体）交配缺陷。通过检查RNA转录本以及测量酵母基因-lacZ融合体的β-半乳糖苷酶活性来分析细胞类型特异性基因表达，结果表明MATa ard1突变体的交配缺陷是由于无法表达MATa细胞交配过程所需的基因。发现MATa细胞中缺乏交配特异性基因表达完全是由于HML位点正常沉默的α信息去抑制所致。在ard1突变体中，HMR位点的隐蔽a信息仅轻微去抑制，其水平不足以影响MATα细胞的交配效率。在HML含有a信息而HMR含有α信息的ard1突变体中，也观察到HML的表达优先于HMR升高。因此，ard1突变的影响是位置特异性的（而非信息特异性的）。尽管ard1突变体的表型类似于SIR1基因突变的细胞，但遗传和生化研究结果均表明，ARD1对HML表达的控制独立于SIR1和其他SIR基因所施加的调控。这些结果表明，ARD1基因编码一种蛋白质产物，该产物直接或间接作用于HML位点以抑制其表达，并且类似地，可能控制参与监测营养状况的其他基因的表达。
