Abstract
The duplication of the nuclear gene specifying the cytosolic isozyme of phosphoglucose isomerase (PGI; EC 5.3.1.9) arose within Clarkia, a genus of annual plants native to California, and now characterizes about half of the diploid species of this genus. Evidence obtained by immunological inhibition and titration of crude leaf extracts demonstrated that species with and without the duplication have the same levels of cytosolic to total PGI (the sum of the cytosolic and plastid PGI activities). The immunological studies were carried out with a specific anticytosolic PGI antiserum and were fully supported by a densitometric analysis of the electrophoretically separated isozymes. Densitometric examination of electrophoretically separated PGIs in 11 vegetable species revealed only two levels of cytosolic to total PGI activities, one of which was the same as in Clarkia. This suggests that only certain levels of the cytosolic isozyme are compatible with proper operation of the cytosolic PGI reaction and make it likely that some form of genic or metabolic regulation has evolved that compensates for the PGI duplication.