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

The effects of ultraviolet irradiation on Escherichia coli 30-S ribosomal subunits were studied. At the doses of radiation used in this work (0–4.5 ⊗ 10⁵ quanta/30-S subunit), only protein S7 was found to be 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 irrradiation with a germicide lamp (254 nm) or with a monochromatic ultraviolet 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 ribonuclease. The oligonucleotide remaining attached to the crosslinked protein was characterised as A-C-C-U-C-G [position 1261–1266, see the sequence published by Carbon et al. (1979) Eur. J. Biochem. 160, 399–410]. Analysis of this fragment suggests that protein S7 was linked to the cytosine at position 1265 in the RNA sequence. [ABSTRACT FROM AUTHOR]