Protein synthesis in Volvox carteri f. nagariensis

Incorporation of radioactive amino acids was used to examine protein synthesis during the asexual reproductive cycle of the HK10 (female) strain of Volvox carteri f. nagariensis. It was observed that arginine is incorporated into TCA-precipitable material 500 to 1000 times as rapidly as other amino acids. Arginine is taken up by the organisms by a saturable transport system and concentrated against a steep concentration gradient. Intracellular arginine is incorporated with high efficiency: after 1 hr, more than 80% of the intracellular radioactivity is in a TCA-precipitable form. N-terminal analysis ruled out the possibility that this tracer arginine was being incorporated via a ribosome-independent arginyl tRNA-protein transferase and supported the alternative explanation that it was being incorporated via conventional protein synthesis. Proteins labeled with arginine were found to decay with the same time course as those labeled with another amino acid.A method for isolation of somatic and reproductive cells free of cross-contamination is reported. Using this method to isolate cells of prelabeled Volvox, it was observed that arginine is preferentially incorporated into reproductive cells. Reproductive cells exhibited four times as high an arginine-to-lysine incorporation ratio as somatic cells. Over half the total incorporated arginine that was recovered was in reproductive cells. Thus arginine incorporation should serve well as a probe to investigate developmentally significant protein synthesis in the reproductive cells of this species.