ACID TOLERANCE IN RHIZOBIUM-MELILOTI STRAIN WSM419 INVOLVES A 2-COMPONENT SENSOR-REGULATOR SYSTEM

An acid-sensitive mutant, TG5-46, derived from Rhizobium meliloti WSM4 19 by Tn5 mutagenesis, fails to grow below ph 6.0 whereas the parent s train grows at ph 5.7. The DNA sequence of a 2.2 kb rhizobial DNA regi on flanking Tn5 in TG5-46 contains two open reading frames, ORF1 (desi gnated actS) and ORF2 (designated actR), having high similarity to the sensor-regulator pairs of the two-component systems involved in signa l transduction in prokaryotes. insertion of an omega interposon into a cfS in R. meliloti WSM419 resulted in an acid-sensitive phenotype. A D NA fragment from the wild-type complemented the acid-sensitive phenoty pe of RT295 (ActS(-)) and TG5-4C (ActR(-)), while fragments containing only actR or actS complemented TG5-4C and RT295, respectively. The pr esence of multiple copies of actR complemented not only TG5-46 but als o RT295. Cloning DNA upstream from actR and actS into a broad-host-ran ge lacZ expression vector and measuring beta-galactosidase activities showed that both genes are constitutively expressed regardless of the external ph. Genomic DNA from all strains of R. meliloti, but no other bacteria tested, hybridized with an actRS probe at high stringency. T hese data implicate a two-component sensor-regulator protein pair in a cid tolerance in R. meliloti and suggest their involvement in ph sensi ng and/or response by these bacteria.