Genetic mapping of the structural gene for phospholipase C of Pseudomonas aeruginosa PAO

Physical mapping of virulence-associated genes in Pseudomonas aeruginosa by transverse alternating-field electrophoresis

The relative chromosomal locations of 20 virulence-associated genes in four clinical isolates of Pseudomonas aeruginosa were investigated by using transverse alternating-field electrophoresis. Each strain had a characteristic restriction pattern when digested with either SpeI or DraI and electrophoresed with 15-s pulses. All four strains had restriction fragments that hybridized with each of the gene probes used, although there were variations in fragment size. An SpeI physical map constructed by Ratnaningsih et al. (E. Ratnaningsih, S. Dharmsthiti, V. Krishnapillai, A. Morgan, M. Sinclair, and B. W. Holloway, J. Gen. Microbiol. 136:2351-2357, 1990) for one of these strains, PAO1, was used to identify the location of 11 previously unmapped genes. The physical locations of the remaining genes were found to be consistent with their genetically mapped loci. Whereas phospholipase C and alginate structural and regulatory genes were associated in three separate clusters in the early, middle, and late regions of the chromosome, no virulence cluster was identified. Our data suggest that the pathogenicity of P. aeruginosa results from the gradual acquisition of genes encoding various virulence determinants.

Purification and some properties of phospholipase C from Achromobacter xylosoxidans

A non-haemolytic phospholipase C (EC 3.1.4.3) was purified from the culture medium of Achromobacter xylosoxidans with a 5% yield and a purification factor of 330. A combination of ultrafiltration, acetone precipitation and two subsequent affinity chromatographic steps was used. The affinity chromatography is a new application of 2-(4-aminophenylsulphonyl)ethyl-cellulose, a sorbent that has previously been used for the purification of phospholipase C from Bacillus cereus. The purified enzyme gave four distinct bands on polyacrylamide gel electrophoresis, and each band was catalytically active. Under our experimental conditions, the phospholipids examined were hydrolysed in the following order: phosphatidylcholine, phosphatidylethanolamine, sphingomyelin. Neither the synthetic substrate p-nitrophenylphosphorylcholine nor phosphatidylinositol was hydrolysed under different experimental conditions. For maximal hydrolytic activity toward phosphatidylcholine, the enzyme required Triton X-100 and Ca2+ ions. EDTA was inhibitory, but the enzyme activity was almost completely restored by Zn2+. The molecular mass of the phospholipase C, estimated by gel permeation, was 34,000 daltons.

Molecular comparison of a nonhemolytic and a hemolytic phospholipase C from Pseudomonas aeruginosa

Pseudomonas aeruginosa produces two secreted phospholipase C (PLC) enzymes. The expression of both PLCs is regulated by Pi. One of the PLCs is hemolytic, and one is nonhemolytic. Low-stringency hybridization studies suggested that the genes encoding these two PLCs shared DNA homology. This information was used to clone plcN, the gene encoding the 77-kilodalton nonhemolytic PLC, PLC-N. A fragment of plcN was used to mutate the chromosomal copy of plcN by the generation of a gene interruption mutation. This mutant produces 55% less total PLC activity than the wild type, confirming the successful cloning of plcN. plcN was sequenced and encodes a protein which is 40% identical to the hemolytic PLC (PLC-H). The majority of the homology lies within the NH2 two-thirds of the proteins, while the remaining third of the amino acid sequence of the two proteins shows very little homology. Both PLCs hydrolyze phosphatidylcholine; however, each enzyme has a distinct substrate specificity. PLC-H hydrolyzes sphingomyelin in addition to phosphatidylcholine, whereas PLC-N is active on phosphatidylserine as well as phosphatidylcholine. These studies suggest structure-function relationships between PLC activity and hemolysis.