An acid-sensitive mutant TG5-46, derived from WSM419 by Tn5 mutagenesis, fails to grow below pH 6.0 whereas the parent strain grows at pH 5.7. The DNA sequence of a 2.2 kb rhizobial DNA region flanking Tn5 in TG5-46 contains two open reading frames, ORF1 (designated ) and ORF2 (designated ), having high similarity to the sensor-regulator pairs of the two-component systems involved in signal transduction in prokaryotes. Insertion of an omega interposon into in WSM419 resulted in an acid-sensitive phenotype. A DNA fragment from the wild-type complemented the acid-sensitive phenotype of RT295 (ActS) and TG5-46 (ActR), while fragments containing only or complemented TG5-46 and RT295, respectively. The presence of multiple copies of complementes not only TG5-46 but also RT295. Cloning DNA upstream from and into a broad-host-range expression vector and measuring β-galactosidase activities showed that both genes are constitutively expressed regardless of the external pH. Genomic DNA from all strains of but no other bacteria tested, hybridized with an probe at high stringency. These data implicate a two-component sensor-regulator protein pair in acid tolerance in and suggest their involvement in pH sensing and/or response by these bacteria.


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