Basis of b-glucosidase multiplicity

--Mahmoud M. Rifaat and Asim Esen

Multiple b-glucosidase electrophoretic variants were resolved from the sporophytic tissues (coleoptiles, roots, scutellum) of maize inbreds. The zymogram presented in part (A) of the Figure indicates the existence of a slower migrating b-glucosidase activity in maize coleoptile extracts (referred to as Glu3-encoded enzyme or activity hereafter) with the following characteristics: (1) electrophoretically invariant regardless of the allelic variant at Glu1, and (2) present in inbreds with Glu1 null activity. These observations suggest that Glu1- and Glu3-encoded activities might not share a subunit in common. Furthermore, activity bands with mobilities intermediate between those of Glu1- and Glu3-encoded activities are detectable in zymograms of inbred genotypes and developed with the sensitive fluorogenic substrate 4-methyl umbelliferyl b-D-glucopyranoside. The electrophoretic mobilities of these intermediate bands are, however, affected proportionately by that of the allelic variant at Glu1 (compare II and FF slots in the Figure). This suggests that these intermediate bands represent hybrid intergenic enzyme molecular forms and that Glu1- and Glu3-encoded enzyme must have different polypeptide subunits. Moreover, the presence of more than one hybrid molecular enzyme form in an inbred genotype rules out the possibility that Glu3-encoded enzyme could be a dimer molecule. The electrophoretic titration curves presented in part (B) of the Figure indicate that Glu1- and Glu3-encoded enzymes have isoelectric points (pI's) of 5 and 8, respectively, with a lower net charge on Glu3-encoded enzyme at most of the pH range. The two activities are further distinguishable by native molecular size (Kr estimated from Ferguson plots are 0.06 and 0.134, for Glu1- and Glu3-activities, respectively), pH optimum, thermal stability, kinetic constants and sensitivity to reducing and sulfhydryl reagents, subunit molecular weight and structure. The occurrence of multiple sporophytic b-glucosidase activities with distinguishable subunit size and structure in inbred genotypes suggests that multiple non-allelic structural genes are encoding maize b-glucosidase multiplicity. Moreover, Glu3-encoded enzyme appears to show little or no activity in Glu1 heterozygous genotypes, with a concomitant absence of its presumptive subunit. Therefore, it appears that the two activities undergo compensatory regulation. The questions currently under investigation are (1) What is the molecular basis of the epistatic interaction between Glu1 and Glu3 genes? (2) Is Glu3 expression determined by a genetic element at Glu1? and (3) Is Glu3 a gene member within a complex Glu1 structure?

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

Return to the MNL 63 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page