FIG. 4.

Lipase secretion and ExeD secretin assembly by A. hydrophila strains expressing substitution mutation derivatives of ExeA. Lipase activity in the culture supernatant of A. hydrophila wild-type strain Ah65 and exeA mutant strain C5.84 complemented with wild-type ExeA (pRJ31.1) or substitution derivatives were assayed as described in Materials and Methods (top). The values of lipase unit per ml per OD₆₀₀ culture supernatant represent the average obtained from three independent experiments for each strain, with standard deviation indicated. The whole-cell samples were analyzed for secretin assembly on 3 to 8% gradient SDS-PAGE gels and immunoblotted with anti-ExeD serum (bottom). The ExeD multimers and monomers are indicated.