The leucine zipper of Aeromonas salmonicida AbcA is required for the transcriptional activation of the P2 promoter of the surface‐layer structural gene, vapA, in Escherichia coli
The Aeromonas salmonicida AbcA protein is involved in the synthesis of the O-polysaccharide side-chains on the lipopolysaccharide and is also capable of enhancing the expression of the structural gene for the A-layer, vapA, when cloned into Escherichia coli. The P2 promoter of the vapA gene of A. salmonicida was cloned into a promoter probe vector and expression in E. coli was monitored. The expression of P2::lacZ was shown to be increased when abcA was provided in trans. AbcA contains an N-terminal ATP-binding domain as well as a C-terminal leucine zipper domain. Site-directed mutagenesis has been used to show that the ATP-binding domain is required for the synthesis of the O-polysaccharide side-chains, but not for the enhancement of vapA expression. Conversely, the leucine zipper is needed for the increase in vapA expression, but not for O-polysaccharide side-chain synthesis. This indicates that AbcA is a bifunctional protein that can influence the synthesis of the two principle antigenic components of the A. salmonicida cell surface.