THE EXTRACELLULAR PROTEIN REGULATOR (XPR) AFFECTS EXOPROTEIN AND AGR MESSENGER-RNA LEVELS IN STAPHYLOCOCCUS-AUREUS

xpr, a regulatory element of exoprotein synthesis in Staphylococcus au reus, defined by an insertion of Tn551 into the chromosome of strain S 6C, affects the expression of several exoproteins at the mRNA level. D rastic reduction in transcript levels for staphylococcal enterotoxin B (seb), lipase (geh), alpha-toxin (hla), and delta-toxin (hld) were de tected, while mRNA levels for coagulase (coa) and protein A (spa) were elevated. Because the delta-toxin gene resides within the RNAIII tran script of the exoprotein regulator, agr, the reduction in hld message in the mutant strain of S6C is indicative of additional regulatory eve nts in exoprotein gene expression. Northern (RNA) analysis of total ce llular RNA hybridized with probes specific for RNAII and RNAIII (the t wo major transcripts of the agr operon) showed that both transcripts w ere reduced 16- to 32-fold at 3 h (late exponential phase) and 8- to 1 6-fold at 12 h (postexponential phase). These data confirm our origina l findings (M. S. Smeltzer, M. E. Hart, and J. J. landolo, Infect. Imm un. 61:919-925, 1993) that two regulatory loci, agr and xpr, are inter active at the genotypic level.