DNA-to-protein Cross-linking in Synchronized HeLa Cells Exposed to Ultra-violet Light

Abstract

The U.V.-induced DNA-to-protein cross-linking in synchronously growing HeLa cells exhibits age-dependent variations. The maximum yield of cross-linking is observed during S-phase and exposure of mid-S cells to 250 ergs/mm² renders about 22 per cent of DNA unextractable owing to cross-linking with proteins. Cells irradiated in G₁ or G₂ exhibit much smaller amount of cross-linking. The unextracted DNA was recovered from the protein fraction, and the addition of 0·5 per cent trypsin solution rendered all the DNA extractable. Fluctuations in the age-response pattern for the yield of DNA-protein cross-links varies throughout the cell-cycle in the same way as the efficiency for cell-killing, i.e. cells are resistant in G₁, sensitive as they move into and through the S phase, and resistant again in G₂. Dose—response curves determined at different times in the cycle reveal that up to about 500 ergs/mm² the amount of DNA recovered from S cells decreased for about 8 per cent for every 100 ergs/mm² of U.V. light. The amount of recovered DNA remained unchanged for cells exposed to 500 ergs/mm² in the middle of S, for as long as 8 hours after exposure, i.e. for as long as the experiments were carried out no recovery from this damage was observed.