Porter S D, Smith M
Nature. 1986;320(6064):766-8. doi: 10.1038/320766a0.
The MAT alpha locus of the yeast Saccharomyces cerevisiae encodes two regulatory proteins, alpha 1 and alpha 2, which are responsible for determining the alpha-cell type. MAT alpha 1 is a positive regulator of alpha-cell-type-specific genes, and MAT alpha 2 is a negative regulator of a-cell-type-specific genes. MAT alpha 2 also determines the a/alpha diploid cell type, in conjunction with the MATa product, a1, by repressing haploid cell-type-specific genes. The MAT alpha 2-encoded protein binds specifically in vitro to a DNA sequence found upstream of several a-specific genes and is thus thought to exert its control directly at the transcriptional level of target genes. In an initial attempt to understand the molecular basis of the interaction of alpha 2 with DNA, we have saturated with missense mutations the segment of alpha 2 that is weakly homologous to a conserved prokaryote DNA-binding structure and to a portion of the higher eukaryote homoeo domain to ascertain the possible functional significance of this homology in alpha 2. We report here that most of the amino-acid residues in alpha 2 which correspond to conserved amino acids in the prokaryote DNA-binding proteins and in the homoeo domain are essential for the two repressor activities of alpha 2, that is, the repression of a-specific genes and of haploid-specific genes. Mutations in a subset of these amino-acid residues more severely affect the ability to repress a-specific genes than haploid-specific genes.
酿酒酵母的MATα基因座编码两种调控蛋白α1和α2，它们负责决定α细胞类型。MATα1是α细胞类型特异性基因的正调控因子，而MATα2是a细胞类型特异性基因的负调控因子。MATα2还与MATa产物a1一起，通过抑制单倍体细胞类型特异性基因来决定a/α二倍体细胞类型。MATα2编码的蛋白在体外特异性结合到几个a特异性基因上游发现的DNA序列上，因此被认为直接在靶基因的转录水平上发挥其调控作用。在初步尝试理解α2与DNA相互作用的分子基础时，我们用错义突变饱和了α2中与保守的原核生物DNA结合结构以及高等真核生物同源结构域的一部分弱同源的片段，以确定这种同源性在α2中可能的功能意义。我们在此报告，α2中与原核生物DNA结合蛋白和同源结构域中的保守氨基酸相对应的大多数氨基酸残基对于α2的两种阻遏活性是必不可少的，即对a特异性基因和单倍体特异性基因的阻遏。这些氨基酸残基子集中的突变对抑制a特异性基因的能力的影响比对单倍体特异性基因的影响更严重。
