Effect of Ssc protein mutations on the outer membrane permeability barrier function in Salmonella typhimurium: a study using ssc mutant alleles made by site-directed mutagenesis.

We have previously discovered and characterized a novel essential enterobacterial protein, the Ssc protein of Salmonella typhimurium and found that the mutation Val291----Met in this protein inhibits bacterial growth at 42 degrees C and the function of its outer membrane permeability barrier at 37 degrees C [7]. In the present paper we prepared, by site-directed mutagenesis, a series of novel plasmid-encoded Ssc mutant proteins and tested their ability to compensate the loss of wild-type Ssc. The mutant proteins Met288----Lys and Gly289----Asp completely lacked this ability, and accordingly, were very defective. Ssc mutants Met288----Leu, Met290----Lys, and Met292----Lys were partially defective. Mutants Met290----Leu and Met292----Leu were non-defective as were also four randomly made mutant proteins with mutations outside the 288-292 region. The S. typhimurium derivative which contained both the chromosomally encoded Ssc Val291----Met and the plasmid-encoded Ssc Gly289----Asp had an outer membrane defect more severe than that caused by SscMet291 only. The mutant Ssc proteins had very little, if any, effect on the outer membrane function in the presence of wild-type Ssc. Even though the function of Ssc is not yet known, our results indicate that region 288-292 is important and that SscAsp289 is thus far the most defective mutant Ssc.