Schurr M J, Martin D W, Mudd M H, Deretic V
Department of Microbiology, University of Texas Health Science Center at San Antonio 78284-7758, USA.
J Bacteriol. 1994 Jun;176(11):3375-82. doi: 10.1128/jb.176.11.3375-3382.1994.
Conversion to mucoidy, caused by the overproduction of the exopolysaccharide alginate in laboratory and cystic fibrosis strains of Pseudomonas aeruginosa, can occur via frameshift or nonsense mutations in the second gene of the algU mucA mucB cluster. The first gene of the cluster, algU, encodes a putative alternative sigma factor required for algD transcription. The algD gene encodes a critical alginate biosynthetic enzyme and is invariably activated in mucoid P. aeruginosa cells. To investigate the function of the genes controlling conversion to mucoidy, the wild-type algU mucA mucB cluster from the standard genetic strain PAO1 was used to reconstitute algD transcription in Escherichia coli. Transcription of an algD-lacZ chromosomal fusion in E. coli was detected upon introduction of plasmid-borne algU mucA mucB. Moreover, insertional inactivation of either mucA or mucB resulted in further stimulation of transcriptional activity from the algD promoter. This activation was dependent on algU, since a double algU mucA mutation abrogated transcription of algD. These experiments suggest that the phenotypic manifestations of muc mutations, i.e., increased algD expression and mucoid phenotype, depend on the presence of an active algU gene and that this regulator and the factors encoded by the downstream genes interact. Further support for these conclusions came from the investigations of the mechanism of reversion to nonmucoidy in P. aeruginosa, a phenomenon frequently referred to as the instability of mucoid phenotype. Spontaneous nonmucoid derivatives of the mucoid strain PAO578 carrying the mucA22 mutation were examined for the presence of alterations within the algU mucA mucB locus. Point mutations which inactivated algU were detected in some, but not all, nonmucoid revertants. No reversion of the original mucA22 mutation (a deletion of one C) was observed in any of the investigated strains. This observation suggests that the process of conversion to nonmucoidy ban be explained, at least partially, by second-site suppressor mutations and that a fraction of such mutations occurs in algU.
在实验室条件下以及铜绿假单胞菌的囊性纤维化菌株中，胞外多糖藻酸盐的过量产生会导致菌株转变为黏液型，这种转变可通过algU mucA mucB基因簇第二个基因中的移码突变或无义突变发生。该基因簇的第一个基因algU编码一种假定的替代sigma因子，它是algD转录所必需的。algD基因编码一种关键的藻酸盐生物合成酶，并且在黏液型铜绿假单胞菌细胞中总是被激活。为了研究控制转变为黏液型的基因的功能，使用来自标准遗传菌株PAO1的野生型algU mucA mucB基因簇在大肠杆菌中重建algD转录。在导入携带algU mucA mucB的质粒后，检测到大肠杆菌中algD - lacZ染色体融合体的转录。此外，mucA或mucB的插入失活导致algD启动子的转录活性进一步增强。这种激活依赖于algU，因为algU mucA双突变消除了algD的转录。这些实验表明，muc突变的表型表现，即algD表达增加和黏液型表型，取决于活性algU基因的存在，并且这种调节因子与下游基因编码的因子相互作用。对铜绿假单胞菌回复为非黏液型机制的研究进一步支持了这些结论，这一现象常被称为黏液型表型的不稳定性。检查了携带mucA22突变的黏液型菌株PAO578的自发非黏液型衍生物在algU mucA mucB基因座内是否存在改变。在一些但不是所有的非黏液型回复株中检测到了使algU失活的点突变。在所研究的任何菌株中均未观察到原始mucA22突变（缺失一个C）的回复。这一观察结果表明，转变为非黏液型的过程至少部分可以由第二位点抑制突变来解释，并且这类突变中有一部分发生在algU中。
