The timing of ribonucleic acid synthesis during the germination of heat-activated Dictyostelium discoideum spores.

To observe changes in the rate of RNA synthesis during Dictyostelium discoideum spore germination, we have pulse labeled germinating spores with [3H]uracil at stage-specific intervals. The labeled RNAs have been fractionated on 1.7 and 10% polyacrylamide gels. Additionally we have monitored the total changes in ribonucleic acid content during germination. In general, the rate of synthesis of various RNA species appears to increase noncoordinately, while the total RNA content remains unchanged during germination. This may indicate extensive RNA turnover during germination. Little or no RNA synthesis is observed during the first pulse period (postactivation lag). During the 1-2-h pulse period, 25, 17, 5.8, and 5 S RNA were all synthesized. The synthesis of these species increased with each pulse period, with the greatest amount of synthesis occurring during the final 3-4-h pulse. The synthesis of 4 S RNA initiated during the 2-3-h pulse and increased during the final 3-4-h pulse. The percentage of poly(A) + RNA increased with each time period from 0.1% of the total labeled RNA at the 0-1-h pulse period to 10% of the total RNA synthesized at the 2-3-h pulse time. During the final pulse period (3-4 h), this level dropped to 6%. Analysis of acid-soluble extracts from dormant spores revealed relatively high levels of the four ribonucleoside 5'-triphosphates. These levels remained unchanged during the 1st h of germination. Thus RNA synthesis during the postactivation lag stage of germination does not appear to be limited by precursor availability and may involve other regulatory mechanisms.