RECOVERY FROM ULTRAVIOLET LIGHT-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 t imes after ultraviolet light (UV) irradiation in normal human, xeroder ma pigmentosum (XP) and Cockayne syndrome (CS) cells. In normal cells, the rate of rRNA synthesis, as measured by the incorporation of H-3-u ridine into 18S and 28S rRNAs, decreased immediately after UV irradiat ion to about half of that of unirradiated cells, and then recovered si gnificantly 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 contra st, group C XP cells showed a slight recovery at 24 h after UV, sugges ting that they have the capacity to repair UV lesions in rRNA genes.