In the Medicago-Rhizobium meliloti symbiosis the bacterial partner is more sensitive to proton stress than its legume partner. The isolation of acid-tolerant strains of R. meliloti like WSM419 from acidic soils of the Mediterranean has led us to investigate the mechanism allowing these strains to function under acid conditions when conventional inoculants do not [3–6].
A two component sensor-regulator pair (ActS/R) is essential for growth of R. meliloti WSM419 at low pH. Mutations in either the sensor protein, ActS, or the regulator, ActR, result in an acid-sensitive phenotype. In some cases, genes controlled by such two component signal transduction systems are adjacent to the sensor-regulator pair [7, 8]. To explore this possibility in strain WSM419 the DNA directly upstream from actS was cloned, mapped and sequenced. Inactivation and expression studies indicate that a presumptive RNA helicase immediately downstream of actR/actS is not controlled by actR. This novel finding is the first report of a helicase in root nodule bacteria.