SUMMARY: The exotoxin A genes from strains PA103 and PAO1 have been independently cloned in a pUC9-derived plasmid. In a non-toxigenic mutant of PAO1 as host, the cloned genes directed the synthesis of intact exotoxin A that expressed ADP-ribosyltransferase activity upon treatment with urea and dithiothreitol. Western-blot analysis of culture supernatants identified a polypeptide of 67 kDa, the molecular mass of intact exotoxin A. There was an approximately 15-fold increase in the toxin yield from cells carrying a cloned PA103 gene compared to PA103, and a 40-fold increase in the yield of toxin from cells carrying a cloned PAO1 gene compared to PAO1; cells carrying the cloned PA103 gene yielded about four times more toxin than those carrying the cloned PAO1 gene. Toxin expression was correlated with the presence of a transcript that was initiated 88 bp upstream from the translational start site. Little or no messenger RNA from either cloned gene could be detected in an , or in a host grown in the presence of 0·1 mMFe, a condition that inhibits toxin expression. The nucleotide sequences of two regions, each of approximately 500 bp, near the 5′ and 3′ termini of the structural gene were established. In these regions, the exotoxin A gene from PAO1 has ten base-pair differences compared to the PA103 gene, three in the non-coding region, and seven in the structural gene, four of which should lead to amino-acid differences. No apparent sequence similarities were found between the inferred promoter region of the exotoxin A gene and that of other genes, nor with the consensus sequence of promoters.


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