Biogenesis of ribosomal RNAs in Dictyostelium discoideum
Final Report Abstract
Most metazoa feature ribosomal DNA (rDNA) clusters that encompass genes for three ribosomal RNA (rRNA) molecules, thereby facilitating efficient transcription. In the ameoba Dictyostelium discoideum, this cluster is contained in extrachromomal DNA palindromes that were thought to originate from a chromosomal copy. This project aimed to elucidate this process, but mainly focused on the biogenesis of mature rRNA molecules from the primary transcript during vegetative growth and development. It came as a surprise that the presumed chromosomal copy of the rDNA cluster does not exist in the genome of the amoeba. Also a hypothesized activity of the RNase DrnB in the rRNA maturation could not be confirmed. We have determined a first model for the secondary structures of the large rRNA molecules in D. discoideum. On this, we have mapped the positions that are methylated, which we have identified using RiboMeth-seq. The snoRNAs that guide the responsible methylation machinery to these positions events were largely identified and further characterized. Their expression levels were shown to vary during the development into spores, which surprisingly, however, did not manifest in varying methylation levels, indicating that these levels are independent of the amounts of the responsible snoRNA. In our future work we aim to advance the model further, mainly by Cryo-EM studies. During development of the amoeba, the accumulation of mature rRNA molecules varies, while their sequence identity remains unchanged. Similar observations were also made for the U3 snoRNA involved in the processing of rRNA precursors as part of the small subunit processosome (SSU). In our ongoing and future work, we will concentrate on the activity of SSU components in the rRNA maturation of D. discoideum.
