Aspartate transcarbamylase activity in etiolated cowpea and soybean hypocotyls infected with cowpea mosaic virus or tobacco ringspot virus

Abstract

Aspartate transcarbamylase activity was measured in buffer extracts of healthy and virus-infected etiolated hypocotyls of cowpea and soybean. Enzyme activity was measured by incorporation of [ 14C]aspartic acid into ureidosuccinate. Products and reactants were separated by Dowex-50 chromatography and identified by thin-layer chromatography. In cowpea hypocotyls is infected with cowpea mosaic virus, up to 5·7, 2·8- and 2·4-fold increases were observed in aspartate transcarbamylase activity, RNA and soluble protein, respectively, and cowpea mosaic virus concentration reached as high as 260 μg/g. In soybean hypocotyls infected with cowpea mosaic virus, aspartate transcarbamylase activity increased only 1·2-fold and cowpea mosaic virus, concentrations was less than 20 μg/g. In soybean hypocotyls infected with tobacco ringspot virus, aspartate transcarbamylase activity increased to 1·6-fold, with significant virus synthesis (130 to 220 μg/g). Aspartate transcarbamylase from healthy cowpea hypocotyls and those infected with cowpea mosaic virus was similarly inhibited by uridine monophosphate. Treating enzymes with uridine monophosphate or mild heat gave no indication that increased aspartate transcarbamylase activity in infected tissues was due to release of pre-existing enzyme from a feedback-inhibited state.