CRITHIDIA LUCILIAE - EFFECT OF PURINE STARVATION ON S-ADENOSYL-L-METHIONINE UPTAKE AND PROTEIN METHYLATION

The utilization of S-adenosyl-L-[methyl-H-3]methionine ([H-3-methyl]Ad oMet) by Crithidia luciliae was assessed under nutrient-replete and pu rine-starvation conditions. Uptake experiments with intact cells demon strated that the radiolabel from this molecule was accumulated by puri ne-starved organisms at a rate similar to 10-fold greater than that ob served in those cultivated in nutrient-replete medium. Purine-starved cells also incorporated the radiolabel into trichloroacetic acid insol uble material at an similar to 10-fold faster rate than nutrient-reple te cells. No differences, however, were observed in the intracellular levels of AdoMet and its metabolites between organisms cultivated unde r the two conditions. Results of comparative labeling studies with [H- 3-methyl]AdoMet, S-adenosyl-L-[carboxyl-C-14]methionine, L-[methyl-H-3 ] methionine and L-[S-35]methionine in the presence and absence of cyc loheximide demonstrated that the incorporation of label from [H-3-meth yl]AdoMet was due to transmethylation and was independent of protein s ynthesis. Further, similar to 15 methylated protein bands were identif ied by SDS-PAGE analysis. Lysates from both purine-starved and nutrien t-replete organisms demonstrated similar levels of activity of three p rotein methyltransferases (PMI, II, III). The differences observed in [H-3-methyl]AdoMet utilization between purine-starved and nutrient-rep lete C. luciliae may reflect the enhanced purine transport capacity wh ich results from purine starvation. (C) 1995 Academic Press, Inc.