Occurrence of Isóenzymes of Glutamine Synthetase in the Alga Chlorella kessleri

Abstract

Two forms (GS₁ and GS₂) of glutamine synthetase have been isolated, separated by ion exchange chromatography, and partly characterized from cells of the green alga Chlorella kessleri. Both forms are present in cells grown autotrophically or heterotrophically on various nitrogen sources, but under all nutritional conditions GS₁ was found to be the major isoenzyme present (60-80%). The activity of both isoenzymes was greatest in cells grown under nitrogen-limiting conditions. Both isoenzymes have molecular weights in the range 340 to 350,000 daltons. GS₁ was found to have a greater thermostability than GS₂: GS₁ was stable at 30°C while GS₂ lost 95% of its activity in 30 minutes. GS₁ was much less sensitive to thiol reactive reagents than GS₂.