ClpXP proteases positively regulate alginate overexpression and mucoid conversion in Pseudomonas aeruginosa

Overproduction of the exopolysaccharide alginate and conversion to a mucoid phenotype in Pseudomonas aeruginosa are markers for the onset of chronic lung infection in cystic fibrosis (CF). Alginate production is regulated by the extracytoplasmic function (ECF) sigma factor AlgU/T and the cognate anti-a factor MucA. Many clinical mucoid isolates carry loss-of-function mutations in mucA. These mutations, including the most common mucA22 allele, cause C-terminal truncations in MucA, indicating that an inability to regulate AlgU activity by MucA is associated with conversion to the mucoid phenotype. Here we report that a mutation in a stable mucoid strain derived from the parental strain PAO1, designated PAO581, that does not contain the mucA22 allele, was due to a single-base deletion in mucA (Delta T180), generating another type of C-terminal truncation. A global mariner transposon screen in PAO581 for non-mucoid isolates led to the identification of three regulators of alginate production, c/pP(PA1801), c/pX(PA1802), and a c/pP paralogue (PA3326, designated c/pP2). The PAO581 null mutants of c/pP, c/PX and c/pP2 showed decreased AlgU transcriptional activity and an accumulation of haemagglutinin (HA)-tagged N-terminal MucA protein with an apparent molecular mass of 15 kDa. The c/pP and c/pX mutants of a CF mucoid isolate revert to the non-mucoid phenotype. The ClpXP and ClpP2 proteins appear to be part of a proteolytic network that degrades the cytoplasmic portion of truncated MucA proteins to release the sequestered AlgU, which drives alginate biosynthesis.