Changes in the complexity of nuclear RNA during development of Dictyostelium discoideum

Abstract

Saturation hybridization with a radioactive single-copy genomic DNA probe has been used to measure the complexity of the total whole cell polyadenylated RNA populations during development of the cellular slime mold Dictyostelium discoideum. The complexity of the polyadenylated nuclear RNA has been deduced by comparison of these numbers with our previous estimates of the complexity of the polyadenylated RNA in the cytoplasm and on polysomes. Assuming asymmetric transcription, 53 to 54% of the single-copy portion of the genome is expressed in polyadenylated RNA in growing and early aggregating cells. Of this only about one-third or 19% is expressed on polysomes as mRNA. By contrast, during late aggregation and culmination, after the time when tips have formed on the aggregates a substantially greater portion of the single-copy genome, 80%, is expressed in the total population of polyadenylated RNA. Again, however, only about one-third or 30% of the single-copy genome is expressed as mRNA. Thus throughout growth and development the complexity of the polyadenylated nuclear RNA is about one and one-half times the complexity of the corresponding mRNA populations. The RNA population from the late developing cells is composed of all those RNA species expressed during growth and early development plus additional RNA species whose expression is specific to late development. Sequences which comprise the majority of the mass of the new polyadenylated RNA transcripts which appear in the cytoplasm and on the polysomes late in aggregation are not detectable in nuclear RNA from growing cells. This suggests that their developmental expression is largely regulated at the level of transcription rather than by controls at the level of processing or transport from the nucleus.