Characteristics of two glutamine synthetase isozymes in soybean

Soybean plants [Glycine max (L.) Merr. cv. Williams] contained two chromatographically distinct isoforms of glutamine synthetase, GS-1 and GS-2. GS-1 was localized in the soluble fraction while GS-2 was in the chloroplasts. GS-2 occurred only in isolated chloroplasts, and etiolation or cycloheximide treatment of light-grown plants resulted in a dramatic decrease of GS-2 activity. Although there were some minor differences in their characteristics, the isoenzymes did not differ significantly. The increase in total activity during the early phases of growth was due largely to the increase in GS-2 activity. The activity in dark-grown plants, when exposed to light, reached almost to the level of light-grown plants within 48 hr. When the two isoenzymes were separated by DEAE-Sephacel chromatography, GS-1 accounted for 20% of total activity recovered. Although this proportion was found in both four- and 16-day-old seedlings, this conclusion may not be extrapolated to intact seedlings because of the activity loss during purification and different stability of the isoenzymes. On a whole seedling basis, the activity of the enzyme increased during the three-week period of germination and early growth. The reverse was true on a unit weight basis. Specific activities of roots and primary stems were much higher than any other parts of the plant.