SUBUNIT ANALYSIS OF GLUTAMINE-SYNTHETASE ISOZYMES FROM ROOT-NODULES OF TEPARY BEAN (PHASEOLUS-ACUTIFOLIUS GRAY)

Native polyacrylamide gel electrophoretic analysis of purified glutamine synthetase (GS) from root nodules of tepary bean (Phaseolus acutifolius Gray) revealed as many as 17 isozymes. The banding of the isozymes showed two distinct regions (designated GS1 and GS2). Analysis by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) demonstrated that the isozymes from each region were composed of two different subunit polypeptides. One of the polypeptides was specific to a particular region, and the other was common to both. The specific polypeptide in GS1 had an isoelectric point (pI) of 6.2 and a molecular weight (mol. wt.) of 43 000 Da. The specific polypeptide in GS2 had a pI of 5.2 and a mol. wt. of 42 000 Da. The common polypeptide had a pI of 5.5 and a mol. wt. of 42 000 Da. When the individual isozymes were cut from the gel and subsequently analyzed by sodium dodecyl sulfate (SDS) PAGE or 2-D PAGE, the results revealed that each isozyme was composed of varying proportions of two of the three polypeptides. GS holoenzyme consists of eight subunit polypeptides. Since the polypeptides from each region were of two kinds, random combination of the polypeptides should generate different isozymes with varying proportions of them. Our findings support this hypothesis.