Molecular cloning, sequencing, and expression of the glutamine synthetase II (glnII) gene from the actinomycete root nodule symbiont Frankia sp. strain CpI1.

In common with other plant symbionts, Frankia spp., the actinomycete N2-fixing symbionts of certain nonleguminous woody plants, synthesize two glutamine synthetases, GSI and GSII. DNA encoding the Bradyrhizobium japonicum gene for GSII (glnII) hybridized to DNA from three Frankia strains. B. japonicum glnII was used as a probe to clone the glnII gene from a size-selected KpnI library of Frankia strain CpI1 DNA. The region corresponding to the Frankia sp. strain CpI1 glnII gene was sequenced, and the amino acid sequence was compared with that of the GS gene from the pea and glnII from B. japonicum. The Frankia glnII gene product has a high degree of similarity with both GSII from B. japonicum and GS from pea, although the sequence was about equally similar to both the bacterial and eucaryotic proteins. The Frankia glnII gene was also capable of complementing an Escherichia coli delta glnA mutant when transcribed from the vector lac promoter, but not when transcribed from the Frankia promoter. GSII produced in E. coli was heat labile, like the enzyme produced in Frankia sp. strain CpI1 but unlike the wild-type E. coli enzyme.