--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?
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