Comparison of two fatty acid synthetases from Euglena gracilis variety bacillaris

Abstract

Two enzyme systems from Euglena gracilis var. bacillaris which catalyze the de novo biosynthesis of fatty acids have been compared. One is a multienzyme complex of high molecular weight which is independent of ACP for activity in vitro, and the other is an ACP-dependent system of discrete enzymes (M. L. Ernst-Fonberg, (1973) Biochemistry 12, 2449–2455). The latter activity is present in small amounts in etiolated cells and increases upon exposure of dark-grown cells to light, while multienzyme complex fatty acid synthetase activity decreases by about one-half after 24 hr of exposure to light. Results from the greening of dark-grown cells in the presence of cycloheximide, chloramphenicol, or spectinomycin suggests that the chloroplast ribosomes are involved in the appearance of the ACP-dependent activity; alternatively, the cytoplasmic ribosomes appear to be the site of biosynthesis of the multienzyme complex fatty acid synthetase (or a protein responsible for its activation). The fatty acid synthetase activities from several chloroplast mutants were measured. The ACP-dependent activity was reduced or not present depending on the degree of impairment of chloroplast development, while the multienzyme complex activity in all instances continued to respond to light or darkness. Antibodies against the purified multienzyme complex extensively inhibited its activity whereas the activity of the ACP-dependent system was consistently stimulated. The two enzyme systems are immunologically cross reactive but not identical.