Abstract
Frankia sp. strain CpI1 has two glutamine synthetases. Glutamine synthetase I (GSI) is present during growth on ammonium or N(2) and is similar to classical prokaryotic glutamine synthetases. Gel-filtration chromatography gave a molecular weight estimate of about 680,000 for the GSI holoenzyme, and denaturing polyacrylamide gel electrophoresis yielded a subunit molecular weight of about 59,000, indicating that GSI is most likely a dodecamer. GSI is regulated by adenylylation, as shown by the presence of two spots on two-dimensional polyacrylamide gel electrophoresis and by its behavior during treatment with snake venom phosphodiesterase. GSII is derepressed during nitrogen starvation and accounts for about 95% of the glutamine synthetase activity in nitrogen-starved cells. It is heat-labile and has a subunit molecular weight of about 43,000. Frankia GSII is similar to GSII enzymes found in all but one member of the Rhizobiaceae analyzed to date. The presence of a second glutamine synthetase in Frankia lends support to the proposal that symbiotic organisms have unique modes of nitrogen nutrition but reopens questions about the origins and uniqueness of GSII genes in members of the Rhizobiaceae.