Protein synthesis during zoospore germination in the aquatic phycomycete Blastocladiella emersonii

Abstract

Protein synthesis is required for germination of nongrowing zoospores of Blastocladiella emersonii into actively growing vegetative cells. This required protein synthesis was found to occur within a 15-min interval, beginning approximately 15 min after germination commenced and ending about 10 min before germination was completed. An increase in the rate of total protein synthesis in this interval was accompanied by an increase in polyribosomes, from 5% or less of total cell ribosomes to about 80%. Actinomycin D did not significantly affect the time at which the rate of protein synthesis increased, estimated from either amino acid incorporation or net polyribosome formation, and had no effect on the kinetics or synchrony of germination, indicating that the amount of mRNA available for translation is initially regulated by posttranscriptional mechanisms. Actinomycin D did, however, reduce the rate of protein synthesis about 2-fold at the earliest time examined and about 10-fold by the completion of germination. The decreasing rate of protein synthesis in the presence of actinomycin D could be correlated with qualitative changes both in polyribosomes and in pulse-labeled proteins. These changes suggest that selective mRNA degradation occurred during the later stages of germination in the presence of actinomycin D.