Recovery from ultraviolet tight-induced depression of ribosomal RNA synthesis in normal human, xeroderma pigmentosum and Cockayne syndrome cells.

The rate of ribosomal RNA (rRNA) synthesis was analyzed at different times after ultraviolet light (UV) irradiation in normal human, xeroderma pigmentosum (XP) and Cockayne syndrome (CS) cells. In normal cells, the rate of rRNA synthesis, as measured by the incorporation of 3H-uridine into 18S and 28S rRNAs, decreased immediately after UV irradiation to about half of that of unirradiated cells, and then recovered significantly at 24 h after UV. However, the rate of synthesis continued to decrease during post-UV incubation in XP cells belonging to groups A, D, E, F and G, as well as in CS cells of groups A and B. In contrast, group C XP cells showed a slight recovery at 24 h after UV, suggesting that they have the capacity to repair UV lesions in rRNA genes.