Ciampi M S, Roth J R
Department of Biology, University of Utah, Salt Lake City 84112.
Genetics. 1988 Feb;118(2):193-202. doi: 10.1093/genetics/118.2.193.
A single site in the middle of the coding sequence of the hisG gene of Salmonella is required for most of the polar effect of mutations in this gene. Nonsense and insertion mutations mapping upstream of this point in the hisG gene all have strong polar effects on expression of downstream genes in the operon; mutations mapping promotor distal to this site have little or no polar effect. Two previously known hisG mutations, mapping in the region of the polarity site, abolish the polarity effect of insertion mutations mapping upstream of this region. New polarity site mutations have been selected which have lost the polar effect of upstream nonsense mutations. All mutations abolishing the function of the site are small deletions; three are identical, 28-bp deletions which have arisen independently. A fourth mutation is a deletion of 16 base pairs internal to the larger deletion. Several point mutations within this 16-bp region have no effect on the function of the polarity site. We believe that a small number of polarity sites of this type are responsible for polarity in all genes. The site in the hisG gene is more easily detected than most because it appears to be the only such site in the hisG gene and because it maps in the center of the coding sequence.
沙门氏菌hisG基因编码序列中间的一个单一位点，是该基因中大多数突变产生极性效应所必需的。hisG基因中位于该位点上游的无义突变和插入突变，对操纵子中下游基因的表达均有强烈的极性效应；位于该位点启动子远端的突变则几乎没有或完全没有极性效应。两个先前已知的hisG突变位于极性位点区域，可消除位于该区域上游的插入突变的极性效应。现已筛选出一些新的极性位点突变，这些突变已丧失上游无义突变的极性效应。所有消除该位点功能的突变均为小缺失；其中三个相同，是独立出现的28个碱基对的缺失。第四个突变是在较大缺失内部的16个碱基对的缺失。该16个碱基对区域内的几个点突变对极性位点的功能没有影响。我们认为，少量这种类型的极性位点负责所有基因中的极性现象。hisG基因中的该位点比大多数位点更容易检测到，因为它似乎是hisG基因中唯一的此类位点，且位于编码序列的中心位置。
