The Regulation of Gene Expression During the Aerobic Germination of Mucor Racemosus Sporangiospores.

This study was undertaken to investigate the regulation of gene expression during the aerobic germination of Mucor racemosus sporangiospores. Sucrose density gradient analysis of spore extracts revealed that the percentage of ribosomes associated with mRNA increased from 23% in dormant spores to 85% after 10 min of germination. L-{(\u2714)C}leucine was immediately incorporated at a rapid rate into protein of a leucine auxotroph, whereas {(\u273)H}uracil or {(\u2732)P}-phosphate were not incorporated into RNA at a significant rate until 20 min after the addition of medium. Newly synthesized RNA did not appear in polysomes until at least 30 min. Polyadenylated RNA, comprising 3.3% of the cellular RNA, was isolated from dormant spores by oligodeoxythymidylic acid-cellulose chromatography. Dormant spore RNA was translated in a rabbit reticulocyte cell free system with an efficiency similar to rabbit globin mRNA. It was concluded that sporangiospores store functional mRNA which is translated immediately upon the addition of nutrient medium. In vivo translation products of the stored mRNA, pulse labeled with L-{(\u2735)S}methionine, were compared to proteins continuously labeled with L-{(\u2735)S}methionine during formation of the spore by analysis on 1- and 2-dimensional polyacrylamide gels. Autoradiography of dried gels revealed several proteins which were synthesized during spore formation but not during early germination. Conversely, other proteins were synthesized during the first 30 min of germination but not during spore formation, even though the mRNA for these proteins must have been synthesized at this time. Spores appear to possess a post-transcriptional regulatory mechanism directing selective translation in the developing spore. The relative rates of synthesis of many proteins were observed to change during the first hour of germination. Many of these changes were accompanied by corresponding changes in the intracellular levels of their respective mRNAs as determined by an in vitro translation assay. Other proteins were identified whose translatable mRNA levels remained constant while in vivo translation rates decreased. These data indicate that sporangiospores can effect qualitative and quantitative changes in gene expression by regulation at both the transcriptional and post-transcriptional levels