Description:
<jats:title>Summary</jats:title><jats:p>The yield of exotoxin A from <jats:italic>Pseudomonas aeruginosa</jats:italic> has been shown to be strain‐dependent. Exotoxin A production requires the presence of the positive regulatory gene, <jats:italic>regA.</jats:italic> We cloned the <jats:italic>regA</jats:italic> genetic locus from the prototypical <jats:italic>P. aeruginosa</jats:italic> strain PAO1 and examined its ability to influence exotoxin A yields compared to the same region cloned from the hyper‐toxin‐producing strain, PA103. The <jats:italic>P. aeruginosa regA</jats:italic> mutant strain, PA103‐29, containing the PAO1 <jats:italic>regA</jats:italic> locus <jats:italic>in trans</jats:italic> produced approximately five to seven times less extracellular exotoxin A than PA103‐29 containing the <jats:italic>regA</jats:italic> locus cloned from the hyper‐toxigenic strain, PA103. Nucleotide sequence analysis of the PAO1 <jats:italic>regA</jats:italic> locus revealed several differences, the most striking of which was the absence of a second open reading frame that was present in the analogous PA103 DNA. In addition, an amino acid substitution was found at position 144 of RegA (Thr in PAO1 and Ala in PA103). Recombinant molecules were constructed to test the contribution of each of these changes in nucleotide sequence on extracellular exotoxin A yields. The amino acid substitution in the PAO1 RegA protein was found not to affect overall exotoxin A yields. In contrast, the presence of the second open reading frame immediately downstream of the PA103 <jats:italic>regA</jats:italic> gene was found to influence extracellular exotoxin A yields. This open reading frame encodes a gene which we call <jats:italic>regB.</jats:italic> Nucleotide sequence analysis indicates that <jats:italic>regB</jats:italic> is 228 nucleotides in length and encodes a protein of 7527 Daltons. Our data suggest that <jats:italic>regB</jats:italic> is required for optimal exotoxin A production and its absence in strain PAO1 partially accounts for the difference in yield of extracellular exotoxin A between P. aeruginosa strains PAO1 and PA103.</jats:p>