RNA polymerase III regulation during mammalian tissue regeneration

There are about 600 loci in the mammalian genome that are annotated as RNA polymerase III genes. These comprise tRNA genes, the genes encoding 5S RNA, the smallest ribosomal RNA, and genes encoding catalytic or structural RNAs involved in processes as diverse as RNA processing or transcription elongation. Most RNA polymerase III genes have similar promoter structures, yet they are transcribed with different efficiencies. Here we have explored how RNA polymerase III occupancy of these genomic loci varies in a normal tissue, the liver, during the transition from a resting state to a proliferating state. We find that after partial hepatectomy, which causes synchronous entry of remaining liver cells into the cell division cycle, there is a tremendous increase in RNA polymerase III occupancy. This increase is, however, not uniform and concerns mostly loci that were lowly occupied by RNA polymerase III in resting liver. The changes in RNA polymerase III occupancy cannot be correlated with changes in RNA polymerase II occupancy around the RNA polymerase III loci nor at nearby RNA polymerase II promoters. RNA polymerase III loci with the largest fold change tend to be located in clusters, with the cluster displaying the largest changes located on chromosome 13. This suggests that increases in RNA polymerase III occupancy during the transition from resting to proliferating state affect mostly genes whose basal rate of transcription is relatively low and which are located in clusters.