A variety of adaptive response systems in prokaryotes often involve two families (two components) of signal transduction proteins, namely, sensory kinases and response-regulators. To extend the list of such sensor/regulator genes for Escherichia coli, we adopted a random screening method in this study. In particular, we isolated a series of recombinant plasmids that are able phenotypically to suppress mutational lesions of both the envZ and phoR/creC genes, each of which encodes a well-characterized sensory-kinase. Among the recombinant plasmids thus isolated, two clones (named pSN11 and pSN25) were subjected to characterization in detail. These analyses allowed us to identify the genetic loci specifying novel members of the sensor/regulator families. One (pSN11) is located around 45 min on the E. coli genetic map, that contains two adjacent coding-sequences (baeS and baeR). The other (pSN25) is located around 93 min of the genetic map, that also comprises two adjacent coding-sequences (basS and basR). These two pairs of gene-products, thus newly identified, were revealed to belong to typical members of the sensor/regulator families. Furthermore, they were demonstrated to exhibit the in vitro phosphotransfer reaction in the presence of ATP, that is also a characteristic of the sensory-kinase and response-regulator proteins.