Ostrovsky de Spicer P, Maloy S
Department of Microbiology, University of Illinois, Urbana 61801.
Proc Natl Acad Sci U S A. 1993 May 1;90(9):4295-8. doi: 10.1073/pnas.90.9.4295.
The proline utilization (put) operon of Salmonella typhimurium is transcriptionally repressed by PutA protein in the absence of proline. PutA protein also carries out the enzymatic steps in proline catabolism. These two roles require different cellular localizations of PutA. Catabolism of proline requires PutA to associate with the membrane because reoxidation of the FAD cofactor in PutA needs the presence of an electron acceptor. Repression of the put operon requires PutA to bind to the put control-region DNA in the cytoplasm. The presence of proline, the inducer, is necessary but not sufficient for PutA to discriminate between its roles as an enzyme or as a repressor. Two conditions that prevent PutA protein binding to the put control region are (i) when proline and an electron acceptor or the cytoplasmic membrane are present or (ii) when PutA is reduced by dithionite. These two conditions increase the relative hydrophobicity of PutA protein, favoring membrane association and therefore enzymatic activity.
鼠伤寒沙门氏菌的脯氨酸利用（put）操纵子在缺乏脯氨酸的情况下会受到PutA蛋白的转录抑制。PutA蛋白还参与脯氨酸分解代谢的酶促步骤。PutA的这两种作用需要其在细胞内有不同的定位。脯氨酸的分解代谢需要PutA与膜结合，因为PutA中FAD辅因子的再氧化需要电子受体的存在。put操纵子的抑制需要PutA在细胞质中与put控制区DNA结合。脯氨酸作为诱导剂的存在对于PutA区分其作为酶或阻遏物的作用是必要的，但并不充分。阻止PutA蛋白与put控制区结合的两个条件是：（i）当脯氨酸和电子受体或细胞质膜存在时；（ii）当PutA被连二亚硫酸盐还原时。这两个条件会增加PutA蛋白的相对疏水性，有利于其与膜结合，从而促进酶活性。
