The synthesis of ribosomal RNA in ultraviolet-irradiated Escherichia coli

Abstract

The effect of ultraviolet irradiation of whole cells of Escherichia coli strain B on RNA synthesis has been studied by fractionation of cell-free extracts in a sucrose gradient in Tris buffer (pH 7·4) and 10 −2 M Mg 2+ in the swinging-bucket rotor No. 25 of the Spinco model-L ultracentrifuge. After irradiation with a dose of 200 erg/mm 2 the synthesis of ribosomes is reduced; the synthesis of 100-70-S and 50-S particles is affected to a greater extent than that of 30-S ribosomes. By fractionation of cell-free extracts in which the protein has been removed by phenol extraction it is shown that the synthesis of 23-S RNA is more sensitive to ultraviolet irradiation than the synthesis of 16-S RNA. The higher radiation sensitivity of the synthesis of the heavier ribosomes in comparison with the synthesis of 30-S ribosomes is explained by the 23-S RNA content of the former.