Virginia Polytech. Institute and State Univ.
--Mahmoud M. Rifaat and Asim Esen
b-glucosidase (b-D-glucoside glucohydrolase, EC. 18.104.22.168) is encoded by the Glu1 locus and its expression exhibits sporophytic specificity. The locus has an unusually high degree of polymorphism, heterozygosity per collection, total panmictic heterozygosity, and null alleles (J. F. Doebley et al., Amer. J. Bot., 1985). The accompanying Figure summarizes the behavior of maize b-glucosidase allozymes encoded by the Glu1 locus. (A) The F1 seedlings (coleoptiles) resulting from crossing two maize inbreds, Oh7BFF and Mo17SS, having electrophoretically distinguishable enzyme variants (fast and slow, respectively), showed three b-glucosidase allozymes; i.e. two parentals and one with an intermediate mass/charge-dependent mobility. Each b-glucosidase allozyme (FF, FS, and SS) revealed a single subunit size of 60kD by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). This supports the dimeric quaternary structure of the enzyme and indicates that Glu1 encodes a polypeptide subunit of about 60kD in size (in preparation). (B) Three b-glucosidase activity bands were resolved from the coleoptilar extracts of all the F1 progeny resulting from crossing normal and null inbreds (Oh7BFF X H95OO and H99II X H95OO). The null inbred H95OO lacks the enzyme subunit (not shown). The slow-migrating activity band (SS) was (1) absent from all parents and common to all F1 progenies resulting from both crosses, and (2) lower in protein amount compared to other bands but indistinguishable from normal Glu1-encoded activities by the criteria of subunit size and peptide structure by limited proteolytic hydrolysis (not shown). This suggested that the null mutation in the inbred H95OO was (1) complemented in trans in the F1 progeny; i.e. due to the lack of a trans-acting regulatory element required for sporophytic expression, and (2) not a true allele to b-glucosidase structural gene. The allelism of this mutation to Glu1 is being re-confirmed since it implies that this trans-acting regulatory element is tightly linked to the locus (pseudoallele).
It is interesting that the mutation in this null inbred is accompanied
by an unexpected expression of Glu1 activity in the pollen grains
(contrary to any normal inbred). This observation, added to the low abundancy
of the autodimer protein (SS) in the sporophyte, suggests that a cis-acting
element might be "abnormally" present at the locus in this inbred and specifies
gametophytic, but limited sporophytic, expression. The possibility that
a single mutational event at or near Glu1 is mediating both the
loss and acquisition of respectively trans- and abnormal cis-acting functions,
is currently under investigation.
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