A REGULATORY CASCADE INVOLVING AARG, A PUTATIVE SENSOR KINASE, CONTROLS THE EXPRESSION OF THE 2'-N-ACETYLTRANSFERASE AND AN INTRINSIC MULTIPLE ANTIBIOTIC-RESISTANCE (MAR) RESPONSE IN PROVIDENCIA-STUARTII

A recessive mutation, aarG1, has been identified that resulted in an 1 8-fold increase in the expression of beta-galactosidase from an aac(2' )-lacZ fusion, Transcriptional fusions and Northern blot analysis demo nstrated that the aarG1 allele also resulted in a large increase in th e expression of aarP, a gene encoding a transcriptional activator of a ac(2')-la, The effects of aarG1 on aac(2')-la expression were mediated by aarP-dependent and -independent mechanisms. The aarG1 allele also resulted in a multiple antibiotic resistance (Mar) phenotype, which in cluded increased chloramphenicol, tetracycline and fluoroquinolone res istance, This Mar phenotype also resulted from aarP-dependent and -ind ependent mechanisms. Sequence analysis of the aarG locus revealed the presence of two open reading frames, designated aarRand aarG, organize d in tandem, The putative AarR protein displayed 75% amino acid identi ty to the response regulator PhoP, and the AarG protein displayed 57% amino acid identity to the sensor kinase PhoQ, The aarG1 mutation, a C to T substitution, resulted in a threonine to isoleucine substitution at position 279 (T279I) in the putative sensor kinase, The AarG produ ct was functionally similar to PhoQ, as it was able to restore wild-ty pe levels of maganin resistance to a Salmonella typhimurium phoQ mutan t, However, expression of the aarP and aac(2')-la genes was not signif icantly affected by the levels of Mg2+ or Ca2+, suggesting that aarG s enses a signal other than divalent cations.