Williams P M, Bannister L A, Redfield R J
Department of Zoology, University of British Columbia, Vancouver, Canada.
J Bacteriol. 1994 Nov;176(22):6789-94. doi: 10.1128/jb.176.22.6789-6794.1994.
The sxy-1 mutation of Haemophilus influenzae causes a 100- to 1,000-fold increase in spontaneous natural competence. We have used mapping and sequencing to identify this mutation as a G-to-A transition in an open reading frame adjacent to the rec-1 locus. This mutation substitutes valine for isoleucine at amino acid 19 of the protein specified by this gene (now named sxy). A multicopy plasmid containing the wild-type sxy gene confers constitutive competence on wild-type cells. Cells carrying this plasmid exhibit, in all stages of growth, DNA uptake levels and transformation frequencies as high those normally seen only after full induction of competence by starvation; deletion of part of the sxy gene from the plasmid abolishes this effect. In contrast, a transposon insertion in sxy entirely prevents both DNA uptake and transformation, indicating that sxy encodes a function essential for competence. These findings suggest that sxy may act as a positive regulator of competence. However, because cells carrying the transposon-inactivated sxy::Tn allele grow slowly under conditions that do not induce competence, sxy may also have a role in noncompetent cells.
流感嗜血杆菌的sxy-1突变导致自然感受态自发水平增加100至1000倍。我们利用定位和测序技术确定该突变是rec-1基因座附近一个开放阅读框中的G到A转换。此突变使该基因（现命名为sxy）所编码蛋白质的第19位氨基酸处的异亮氨酸被缬氨酸取代。含有野生型sxy基因的多拷贝质粒赋予野生型细胞组成型感受态。携带该质粒的细胞在生长的各个阶段都表现出与通常仅在饥饿充分诱导感受态后才出现的高水平DNA摄取和转化频率；从质粒中删除部分sxy基因可消除这种效应。相反，sxy中的转座子插入完全阻止了DNA摄取和转化，表明sxy编码感受态所必需的功能。这些发现表明sxy可能作为感受态的正调控因子发挥作用。然而，由于携带转座子失活的sxy::Tn等位基因的细胞在不诱导感受态的条件下生长缓慢，sxy在非感受态细胞中可能也有作用。
