Characterization and regulation of the Pseudomonas aeruginosa algC gene encoding phosphomannomutase

The nucleotide sequence of the Pseudomonas aeruginosa algC gene encoding phosphomannomutase (PMM; EC 5.4.2.8) was determined. The codon usage in algC in the wobble base position was 90.4% G+C, typical of Pseudomonas genes. The predicted amino acid sequence of phosphomannomutase (PMM) showed homology over a stretch of 112 amino acids in the carboxyl terminus with rabbit muscle phosphoglucomutase (PGM), an enzyme that catalyzes a reaction analogous to the catalyzed by PMM. In addition, a specific amino acid sequence within PMM showed homology with the catalytic site of PGM. DNA sequence analysis of a defective algC gene (algC') cloned from a mutant of P. aeruginosa that lacked PMM activity revealed one point mutation (a C to T transition) in the carboxyl terminus of PMM which resulted in an amino acid change from arginine 420 to cysteine 420. The mutation identified in the algC' gene was not within the regions of homology with PGM. The algC promoter showed significant homology with the promoters of two other P. aeruginosa genes involved in alginate synthesis, algD and algR1. Both the algD and algR1 promoters are activated by the product of the algR1 gene in P. aeruginosa. The upstream region of the algC gene contained a sequence identical to the algD upstream sequence that is known to be the binding site for the AlgR1 protein. Expression of algC was reduced 5.7-fold in an algR1 mutant of P. aeruginosa compared to its isogenic parent strain (lacking the algR1 mutation), suggesting that the algR1 gene product activates the transcription of the algC gene.