Huang J H, Schell M A
Department of Microbiology, University of Georgia, Athens 30602.
J Bacteriol. 1990 Jul;172(7):3879-87. doi: 10.1128/jb.172.7.3879-3887.1990.
The pglA gene encodes a 52-kilodalton extracellular polygalacturonase (PGA) which is associated with the phytopathogenic virulence of Pseudomonas solanacearum. The nucleotide sequence of pglA and the putative amino acid sequence of the PGA protein were determined. A computer search identified a 150-residue region of PGA which was similar (41%) to the amino acid sequence of a region of the PG-2A polygalacturonase from tomato. Comparison of the amino terminus of the pglA open reading frame with the actual amino-terminal sequence of purified extracellular PGA suggested that pglA is initially translated as a higher-molecular-mass precursor with a 21-residue amino-terminal signal sequence. Localization of various pglA-phoA fusion proteins in Escherichia coli and P. solanacearum indicated that the 21-residue leader sequence directs the export of PhoA only as far as the periplasm of both bacteria. Deletion of the last 13 residues of PGA eliminated its catalytic activity, as well as its ability to be exported outside of the P. solanacearum cell. Our results suggest that PGA excretion occurs in two steps. The first step involves a signal sequence cleavage mechanism similar to that used for periplasmic proteins and results in export of PGA across the inner membrane; the second step (transit of the outer membrane) occurs by an unknown mechanism requiring sequences from the mature PGA protein and biochemical factors absent from E. coli.
pglA基因编码一种52千道尔顿的细胞外多聚半乳糖醛酸酶（PGA），该酶与青枯假单胞菌的植物致病毒力相关。测定了pglA的核苷酸序列以及PGA蛋白的推测氨基酸序列。计算机搜索鉴定出PGA的一个150个残基的区域，该区域与番茄PG-2A多聚半乳糖醛酸酶一个区域的氨基酸序列相似（41%）。将pglA开放阅读框的氨基末端与纯化的细胞外PGA的实际氨基末端序列进行比较，结果表明pglA最初被翻译为一种具有21个残基氨基末端信号序列的高分子量前体。各种pglA-phoA融合蛋白在大肠杆菌和青枯假单胞菌中的定位表明，21个残基的前导序列仅将PhoA引导至两种细菌的周质。删除PGA的最后13个残基消除了其催化活性以及其被分泌到青枯假单胞菌细胞外的能力。我们的结果表明，PGA的分泌分两步进行。第一步涉及一种类似于用于周质蛋白的信号序列切割机制，导致PGA穿过内膜分泌；第二步（穿过外膜）通过一种未知机制发生，该机制需要成熟PGA蛋白的序列以及大肠杆菌中不存在的生化因子。
