Martin D W, Schurr M J, Mudd M H, Govan J R, Holloway B W, Deretic V
Department of Microbiology, University of Texas Health Science Center, San Antonio 78284-7758.
Proc Natl Acad Sci U S A. 1993 Sep 15;90(18):8377-81. doi: 10.1073/pnas.90.18.8377.
Chronic respiratory infections with mucoid Pseudomonas aeruginosa are the leading cause of high mortality and morbidity in cystic fibrosis (CF). The initially colonizing strains are nonmucoid, but in the CF lung they invariably convert into the mucoid, exopolysaccharide alginate-overproducing form causing further deterioration and poor prognosis. Here we report the molecular basis of conversion to mucoidy. The algU gene is required for expression of the key alginate biosynthetic gene algD and encodes a protein homologous to sigma H, an alternative sigma factor regulating sporulation and other post-exponential-phase processes in Bacillus. The algU gene and the negative regulators mucA and mucB constitute the gene cluster controlling conversion to mucoidy. We demonstrate a critical role of mucA in this process based on (i) the presence of frameshift mutations disrupting the mucA coding region in mucoid cells that were absent in nonmucoid parental strains, (ii) genetic complementation of mucA mutations with the mucA+ gene, (iii) allelic replacements with specific mutant mucA genes causing conversion to mucoidy in previously nonmucoid cells, and (iv) detection of identical and additional mucA mutations in clinical mucoid strains isolated from the lungs of CF patients. These results suggest that the switch from the nonmucoid to mucoid state can be caused by inactivation of mucA, resulting in constitutive expression of alginate biosynthetic genes dependent on algU for transcription and that such mutants may be selected in vivo during chronic infections in CF.
黏液型铜绿假单胞菌引起的慢性呼吸道感染是囊性纤维化（CF）患者高死亡率和高发病率的主要原因。最初定植的菌株是非黏液型的，但在CF患者的肺部，它们总是会转变为黏液型，即过量产生胞外多糖藻酸盐的形式，从而导致病情进一步恶化和预后不良。在此，我们报告了细菌转变为黏液型的分子基础。藻酸盐生物合成关键基因algD的表达需要algU基因，该基因编码一种与σH同源的蛋白，σH是一种替代的σ因子，可调节芽孢杆菌的孢子形成及其他指数期后过程。algU基因与负调控因子mucA和mucB构成了控制转变为黏液型的基因簇。基于以下几点，我们证明了mucA在这一过程中的关键作用：（i）黏液型细胞中存在移码突变，破坏了mucA编码区，而非黏液型亲代菌株中不存在这种突变；（ii）用mucA+基因对mucA突变进行遗传互补；（iii）用特定的突变mucA基因进行等位基因替换，可使先前的非黏液型细胞转变为黏液型；（iv）在从CF患者肺部分离出的临床黏液型菌株中检测到相同及其他的mucA突变。这些结果表明，从非黏液型到黏液型状态的转变可能是由mucA失活引起的，导致依赖algU转录的藻酸盐生物合成基因组成型表达，并且这种突变体可能在CF慢性感染期间在体内被选择出来。
