The roles of the rel+ gene and of 4-thiouridine in killing and photoprotection of Escherichia coli by near-ultraviolet radiation

Abstract— Photoprotection is a reduction in response to far-UV (190–300. nm) radiation in cells that have been previously exposed to longer wavelengths. It has been proposed that photoprotection operates by means of a growth delay that permits more time for dark repair. Growth delay in Escherichia coli utilizes 4-thiouridine (⁴Srd) in transfer RNA as a chromophore and it requires the rel⁺ gene, which exerts a stringent control upon RNA synthesis. Mutants that were either rel or ⁴Srd^? were isolated from E. coli B, utilizing a near-UV-induced growth-delay selection technique. The rel mutants, which undergo little growth delay after near-UV irradiation, show only 50% as much photoprotection as wild types, while ⁴Srd^? mutants show no photoprotection at all. Thus, photoprotection appears to utilize ⁴Srd as its sole chromophore in E. coli B and B/r, and no more than 50% of photoprotection in these strains can be a result of near-UV-induced growth delay.