Abstract
Increased sensitivity to killing by human serum complement occurs in Salmonella typhimurium strains in which the 95-kilobase virulence plasmid is integrated into the chromosome. This phenotypic change appears to be due to alterations in plasmid gene expression and is reversed by the presence of an autonomous plasmid bearing a cloned region of the virulence plasmid. Accordingly, this region has been termed rsk for reduced serum killing. Sequence analysis of the region reveals that rsk is composed of a series of direct 10-base-pair (bp) repeats with a 21-nucleotide periodicity. Two adjacent repeats are identical, but increasing loss of conservation is apparent with increased distance both 5' and 3' of these highly conserved 10-mers. The smallest isolated sequence which restores the serum-resistant phenotype is only 66 bp long and contains the two identical 10-mers and one degenerate 10-mer (8 of 10 bp conserved) 3' of these. The minimal rsk region of 66 bp does not appear to contain a coding sequence, or a promoter, for a structural gene. It is proposed that the minimal rsk is an isolated regulatory site involved in the regulation of the serum resistance of S. typhimurium. Integration of the 95-kilobase plasmid disrupts the normal regulation of virulence plasmid genes, resulting in an increase in the killing of the bacteria by complement activated by the classical pathway. The introduction of the minimal rsk on a multiple-copy plasmid restores resistance to serum killing, possibly through the titration of a trans-acting regulatory factor.