Effects of inhibitors of RNA and protein synthesis on aspartate transcarbamylase activity in etiolated plant tissue

Aspartate transcarbamylase (ATCase) activity declines in etiolated cowpea (Vigna unguiculata L. Walp.) and soybean (Glycine max L. Merr.) hypocotyls between 3 and 11 days after planting. Treating cow-pea hypocotyls with cycloheximide (CH), actinomycin D (AMD), 6-methyl purine (6-MP), or cordycepin increases ATCase activity up to 740, 350, 465, and 305%, respectively, over water-treated controls 48 to 72 hours after treatment. In contrast erythromycin had no effect, and d-threo-chloramphenicol (CHL) reduced ATCase activity nearly 40%. CH, AMD, and CHL, whose effects were further characterized, each markedly reduced total RNA synthesis and protein synthesis. Respiration was stimulated by CH and AMD and reduced by CHL. In soybean, CHL-treated tissues and water-treated controls had comparable ATCase activities 48 hours after treatment, while AMD, 6-MP, and CH treatments reduced activities 29, 37, and 78%, respectively. The results suggest that the level of ATCase activity in etiolated cowpea hypocotyls is regulated by a mechanism or mechanisms that are interfered with by inhibition of RNA and protein synthesis. Possibly the mechanism is absent from etiolated soybean hypocotyls.