Schwacha A, Bender R A
Department of Biology, University of Michigan, Ann Arbor 48109.
J Bacteriol. 1990 Sep;172(9):5477-81. doi: 10.1128/jb.172.9.5477-5481.1990.
The hutC gene of Klebsiella aerogenes encodes a repressor that regulates expression of the histidine utilization (hut) operons. The DNA sequence of a region known to contain hutC was determined and shown to contain two long rightward-reading open reading frames (ORFs). One of these ORFs was identified as the 3' portion of the hutG gene. The other ORF was the hutC gene. The repressor predicted from the hutC sequence contained a helix-turn-helix motif strongly similar to that seen in other DNA-binding proteins, such as lac repressor and the catabolite gene activator protein. This motif was located in the N-terminal portion of the protein, and this portion of the protein seemed to be sufficient to allow repression of the hutUH operon but insufficient to allow interaction with the inducer. The presence of a promoterlike sequence and a ribosome-binding site immediately upstream of the hutC gene explained the earlier observation that hutC can be transcribed independently of the other hut operon genes. The predicted amino acid sequence of hut repressor strongly resembled that of the corresponding protein from Pseudomonas putida (S. L. Allison and A. T. Phillips, J. Bacteriol. 172:5470-5476, 1990). An unexpected, leftward-reading ORF extending from about the middle of hutC into the preceding (hutG) gene was also detected. The deduced amino acid sequence of this leftward ORF was quite distinct from that of an unexpected ORF of similar size found immediately downstream of the P. putida hutC gene. The nonstandard codon usage of this leftward ORF and the expression of repressor activity from plasmids with deletions in this region made it unlikely that this ORF was necessary for repressor activity.
产气克雷伯菌的hutC基因编码一种阻遏蛋白，该蛋白调控组氨酸利用（hut）操纵子的表达。已测定了已知包含hutC的一个区域的DNA序列，结果显示该区域含有两个向右读码的长开放阅读框（ORF）。其中一个ORF被鉴定为hutG基因的3'部分。另一个ORF就是hutC基因。从hutC序列预测的阻遏蛋白含有一个螺旋-转角-螺旋基序，与其他DNA结合蛋白（如乳糖阻遏蛋白和分解代谢基因激活蛋白）中所见的基序非常相似。该基序位于蛋白质的N端部分，该部分蛋白质似乎足以抑制hutUH操纵子，但不足以与诱导物相互作用。hutC基因上游紧邻一个启动子样序列和一个核糖体结合位点，这解释了早期观察到的hutC可以独立于其他hut操纵子基因进行转录的现象。hut阻遏蛋白的预测氨基酸序列与恶臭假单胞菌相应蛋白的序列非常相似（S. L. 艾利森和A. T. 菲利普斯，《细菌学杂志》172:5470 - 5476，1990）。还检测到一个意想不到的向左读码的ORF，它从hutC中部左右延伸至前面的（hutG）基因。这个向左ORF的推导氨基酸序列与恶臭假单胞菌hutC基因下游紧邻的一个大小相似的意想不到的ORF的序列截然不同。该向左ORF的非标准密码子使用情况以及该区域有缺失的质粒所表达的阻遏蛋白活性表明，这个ORF不太可能是阻遏蛋白活性所必需的。
