Le S, Davis C, Konopka J B, Sternglanz R
Department of Biochemistry and Cell Biology, State University of New York at Stony Brook 11794, USA.
Yeast. 1997 Sep 15;13(11):1029-42. doi: 10.1002/(SICI)1097-0061(19970915)13:11<1029::AID-YEA160>3.0.CO;2-1.
Two new yeast genes, ASF1 (Anti-Silencing Function) and ASF2, as well as a C-terminal fragment of SIR3, were identified as genes that derepressed the silent mating type loci when overexpressed. ASF2 overexpression caused a greater derepression than did ASF1. ASF1 overexpression also weakened repression of genes near telomeres, but, interestingly, ASF2 had no effect on telomeric silencing. Sequences of these two genes revealed open reading frames of 279 and 525 amino acids for ASF1 and ASF2, respectively. The ASF1 protein was evolutionarily conserved, MCB motifs, sequences commonly present upstream of genes transcribed specifically in S phase, were found in front of both genes, and, indeed, both genes were transcribed specifically in the S phase of the cell cycle. While an asf2 mutant was viable and had no obvious phenotypes, an asf1 mutant grew poorly. Neither mutant exhibited derepression of the silent mating type loci. The asf1 mutant was sensitive to methyl methane sulfonate, slightly UV-sensitive and somewhat deficient in minichromosome maintenance. It also lowered the restrictive temperature of a cdc13ts mutant. These phenotypes suggested a role for ASF1 in DNA repair and chromosome maintenance.
两个新的酵母基因ASF1（抗沉默功能）和ASF2，以及SIR3的一个C末端片段，被鉴定为在过表达时能解除沉默交配型基因座抑制的基因。ASF2过表达比ASF1过表达导致的抑制解除作用更强。ASF1过表达也减弱了端粒附近基因的抑制作用，但有趣的是，ASF2对端粒沉默没有影响。这两个基因的序列分别显示ASF1和ASF2的开放阅读框为279和525个氨基酸。ASF1蛋白在进化上是保守的，在这两个基因之前都发现了MCB基序，即通常存在于S期特异性转录基因上游的序列，实际上，这两个基因都在细胞周期的S期特异性转录。虽然asf2突变体是有活力的且没有明显表型，但asf1突变体生长不良。两个突变体均未表现出沉默交配型基因座的抑制解除。asf1突变体对甲磺酸甲酯敏感，对紫外线略有敏感，并且在微型染色体维持方面有所缺陷。它还降低了cdc13ts突变体的限制温度。这些表型表明ASF1在DNA修复和染色体维持中起作用。
