RNA SYNTHESIS IN ESCHERICHIA COLI AFTER IRRADIATION WITH ULTRAVIOLET LIGHT

Escherichia coli B and derivatives with different sensitivity to ultraviolet light ( B r, B, Bs - 1) were irradiated with u.v. doses ranging from 500 to 10,000 erg/mm2. Post-irradiation RNA synthesis was measured, and the sedimentation distributions of RNA synthesized after irradiation were determined. The following conclusions were drawn. (1) Synthesis of RNA molecules is terminated and RNA polymerase molecules are liberated at the site of u.v. lesions on the DNA. (2) A dose of 1000 erg/mm2 produces about one transcription-terminating lesion per 1000 DNA base pairs in all of the bacterial strains used. This number is approximately equal to one-half of the number of pyrimidine dimers reportedly produced by that dose, suggesting that every pyrimidine dimer located on the functional DNA strand stops transcription. (3) Within 45 minutes after irradiation (dose greater than 1000 erg/mm2) the RNA synthesis rate in the bacterial strains tested is not significantly increased by photo- or dark repair under the conditions used. (4) With increasing u.v. dose, the rate of RNA chain initiation (number of molecules produced per unit time) is reduced. This reduction may be caused by irreversible binding or delayed liberation of RNA polymerase molecules at u.v. lesions. (5) Most fragments and some complete molecules of ribosomal RNA synthesized after u.v. irradiation are broken down within 45 minutes after synthesis. © 1969.