Isolation and characterization of extragenic suppressor mutants of the tolA-876 periplasmic-leaky allele in Escherichia coli K-12.

tolA mutants of Escherichia coli K-12 release periplasmic proteins into the extracellular medium; they are sensitive to growth inhibitors such as cholic acid and tolerant to group A colicins and filamentous bacteriophage. Suppressor mutants of the tolA-876 allele were isolated by selecting for cholic acid resistant clones that did not release periplasmic ribonuclease I. One class of tolA suppressor strains carried mutations in the staA gene (for suppressor of tolA) located a 41 min. tolA-876 staA strains partially recovered a wild-type phenotype: they exported alkaline phosphatase and beta-lactamase into the periplasm and only released very low amounts of periplasmic proteins; moreover, they were sensitive to E1 and A colicins and more resistant than tolA-876 staA+ strains to various growth inhibitors. Furthermore, tolA-876 staA-2 and tolA+staA-2 mutants were 10- to 2700-times more resistant than staA+ strains to bacteriophages TuIa, TuIb and T4, and TuII whose receptors are major outer membrane proteins OmpF, OmpC and OmpA, respectively. SDS-PAGE analysis suggested that cell envelopes of staA or staA+ strains contained similar amounts of these proteins but characterization of strains carrying ompF (or C or A)-phoA gene fusions showed that mutation stA-2 reduced ompF gene expression by a factor of two. Analysis of double mutants strains carrying mutation staA-2 and a tolA, tolB, excC or excD periplasmic-leaky mutation showed that staA suppression was allele specific which suggested that proteins TolA and StaA might directly interact.