Differential regulation of S-adenosylmethionine synthetase isozymes by gibberellic acid in dwarf pea epicotyls.

Dwarf pea epicotyls contained a single activity peak of S-adenosylmethionine (AdoMet) synthetase (isozyme I). Gibberellic acid (GA3, 1 microM) induced two additional isozymes (II and III). Cycloheximide (20 microgram/ml) blocked the appearance of GA3-induced isozymes, suggesting that it is dependent on de novo protein synthesis. Conclusive proof was obtained by labelling the isozymes II and III with [35S]SO2-(4) in vivo. The purified 35S-labelled AdoMet synthetase isozymes (II, III) showed a single protein band that coincided with the single radioactive peak on SDS-PAGE. Molecular-sieve chromatography of the isozyme I from control dwarf pea epicotyls and three isozymes of GA3-treated epicotyls on Sepharose CL-6B showed a single activity peak with an identical molecular mass of 174 kDa for each isozyme. Analysis of purified AdoMet synthetase isozymes (I, II, III) on SDS-PAGE showed a single silver-stained protein band with a molecular mass of 87 kDa. This proved the dimeric nature of all the isozymes of AdoMet synthetase which could be physically separated by ion-exchange chromatography on DE-52. In vitro molecular hybridization of physically separated isozymes by NaCl-freeze-thaw treatment method revealed that the three isozymes (I, II, III) in GA3-treated dwarf pea epicotyls are formed through the random dimerization of two different species of enzyme subunits that differ in their net charge. Thus, the two flanking activity peaks (isozymes I, III) represent homodimers, while the middle activity peak (isozyme II) is a heterodimer. Apparently, the single isozyme I in control epicotyls is a product of one gene of AdoMet synthetase (SAM 1), while three isozymes in GA3-treated epicotyls are the product of two genes of AdoMet synthetase. We speculate that the differential regulation of AdoMet synthetase in GA3-treated epicotyls is achieved by the expression of an alternate gene of AdoMet synthetase (SAM 2).