D'Elia J N, Salyers A A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
J Bacteriol. 1996 Dec;178(24):7180-6. doi: 10.1128/jb.178.24.7180-7186.1996.
Bacteroides thetaiotaomicron, a gram-negative obligate anaerobe, appears to utilize starch by first binding the polymer to its surface and then translocating it into the periplasmic space. Several genes that encode enzymes or outer membrane proteins involved in starch utilization have been identified. These have been called sus genes, for starch utilization system. Previous studies have shown that sus structural genes are regulated at the transcriptional level and their expression is induced by maltose. We report here the identification and characterization of a gene, susR, which appears to be responsible for maltose-dependent regulation of the sus structural genes. The deduced amino acid sequence of SusR protein had a helix-turn-helix motif at its carboxy-terminal end, and this region had highest sequence similarity to the corresponding regions of known transcriptional activators. A disruption in susR eliminated the expression of all known sus structural genes, as expected if susR encoded an activator of sus gene expression. The expression of susR itself was not affected by the growth substrate and was not autoregulated, suggesting that binding of SusR to maltose might be the step that activates SusR. Three susR-controlled structural genes, susA, susB, and susC, are located immediately upstream of susR. These genes are organized into two transcriptional units, one containing susA and another containing susB and susC. susA was expressed at a lower level than susBC, and susA expression was more sensitive to the gene dosage of susR than was that of the susBC operon. An unexpected finding was that increasing the number of copies of susR in B. thetaiotaomicron increased the rate of growth on starch. This effect could be due to higher levels of susA expression. Whatever the explanation, the level of SusR in the cell appears to be a limiting factor for growth on starch.
脆弱拟杆菌是一种革兰氏阴性专性厌氧菌，它似乎通过先将淀粉聚合物结合到其表面，然后将其转运到周质空间来利用淀粉。已经鉴定出几个编码参与淀粉利用的酶或外膜蛋白的基因。这些基因被称为sus基因，即淀粉利用系统。先前的研究表明，sus结构基因在转录水平上受到调控，其表达由麦芽糖诱导。我们在此报告一个基因susR的鉴定和特征，它似乎负责sus结构基因的麦芽糖依赖性调控。SusR蛋白推导的氨基酸序列在其羧基末端有一个螺旋-转角-螺旋基序，该区域与已知转录激活因子的相应区域具有最高的序列相似性。正如susR编码sus基因表达激活因子时所预期的那样，susR的破坏消除了所有已知sus结构基因的表达。susR自身的表达不受生长底物的影响，也不受自身调控，这表明SusR与麦芽糖的结合可能是激活SusR的步骤。三个受susR控制的结构基因susA、susB和susC位于susR的紧上游。这些基因被组织成两个转录单元，一个包含susA，另一个包含susB和susC。susA的表达水平低于susBC，并且susA的表达比susBC操纵子对susR的基因剂量更敏感。一个意外的发现是，增加脆弱拟杆菌中susR的拷贝数会提高在淀粉上的生长速率。这种效应可能是由于susA表达水平较高。无论原因是什么，细胞中SusR的水平似乎是淀粉生长的限制因素。
