Effect of Ultraviolet Irradiation on 30‐S Ribosomal Subunits. Identification of the RNA Region Crosslinked to Protein S7

Abstract
The effects of UV irradiation on Escherichia coli 30-S ribosomal subunits were studied. At the present doses of radiation (0-4.5 .times. 105 quanta/30-S subunit), only protein S7 was significantly crosslinked to the 16-S RNA. In conditions where 25% of the protein was covalently crosslinked, the ability of the irradiated 30-S subunits to reassociate with 50-S subunits and their activity in polyphenylalanine synthesis decreased strongly. Similar results were obtained by irradiation with a germicide lamp (254 nm) or with a monochromatic UV light at 248 nm. No additional proteins were crosslinked to the 16-S RNA by irradiating 30-S subunits depleted in protein S1 or 70-S ribosomes. The covalent complex of 16-S RNA and protein S7 was isolated and digested by T1 RNase. The oligonucleotide remaining attached to the crosslinked protein was characterized as A-C-C-U-C-G [position 1261-1266]. Analysis of this fragment suggests that protein S7 was linked to the cytosine at position 1265 in the RNA sequence.