Role of SspA in the density‐dependent expression of the transcriptional activator AarP in Providencia stuartii

The AarP protein in Providencia stuartii encodes a small transcriptional activator which activates the chromosomal aminoglycoside acetyltransferase aac ( 2 ′)‐ Ia gene. In addition, AarP activates genes involved in a multiple antibiotic resistance (Mar) phenotype. Expression of an aarP–lacZ fusion increased in a density‐dependent manner and reached peak levels at stationary phase. The expression of an aarP–lacZ fusion could be prematurely activated in cells at early to mid‐exponential phase by the addition of spent culture supernatants from stationary phase cultures or by ethyl acetate extracts of these supernatants. Nutrient starvation had a negligible effect on aarP expression. In a search for mutations that block aarP activation at stationary phase, a mini‐Tn 5Cm insertion has been identified within a gene whose product was 77% identical to SspA, a regulatory protein involved in stationary phase gene expression and virulence. An unmarked sspA null allele ( sspA2 ) was created by allelic replacement to further examine the role of sspA in P. stuartii . The sspA2 allele resulted in substantial decrease in aarP mRNA accumulation at various phases of growth. Furthermore, in an sspA mutant background, the aarP–lacZ fusion was no longer activated by an extracellular signal.