A HELICASE GENE (HELO) IN RHIZOBIUM-MELILOTI WSM419

A 2.8 kb BamHI DNA fragment adjacent to a BamHI fragment containing ac tR-actS (a sensor/regulator pair required for low pH tolerance in Rhiz obium meliloti WSM419) was cloned and sequenced. A computer predicted protein of 821 amino acids, designated HelO, showed extensive similari ty with 'DEAH' motif helicases. Expression of helO was higher at pH 7. 0 than pH 5.8 and it did not require the product of the actR gene. Ina ctivation of helO by insertion of a Omega interposon at codon 40 did n ot affect nodulation, growth or tolerance to low pH, high temperature, osmolarity or elevated levels of copper or zinc.