Stored mRNA in sporangiospores of the fungus Mucor racemosus

Abstract

When introduced into nutrient medium under air, the asexual sporangiospores of Mucor racemosus germinated with 5 to 8 h, culminating with the emergence of germ tubes. We found that sporangiospores increased 20% in dry weight during the first 60 min of germination, indicating a high degree of synthetic activity. Sucrose density gradient analysis of spore extracts revealed that the percentage of ribosomes associated with mRNA increased from 22.5% in dormant spores to 85% within 10 min after the addition of medium and remained at this level for at least 3 h. L-[14C]leucine was immediately incorporated at a rapid rate into protein of a leucine auxotroph, whereas [3H]uracil or [32P]phosphate was incorporated into RNA at a significant rate only 20 min after the addition of medium. This newly synthesized RNA occurred in polysomes only after 30 min had passed. Pool synthesized RNA occurred in polysomes only after 30 min had passed. Pool equilibration of the radioactive precursors was not limiting to these measurements. Polyadenylated RNA was isolated from dormant spores by oligodeoxythymidylic acid-cellulose chromatography and was found to comprise 3.3% of the total cellular RNA. Sucrose density gradient centrifugation revealed the polyadenylated RNA to be heterodisperse in size, ranging from 6S to 20S. It was concluded that M. racemosus sporangiospores contain preformed mRNA which is translated commencing immediately upon the addition of nutrient medium.