4-Thiouridine Triggers Both Growth Delay Induced by Near-Ultraviolet Light and Photoprotection

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Abstract
4-Thiouridine, a rare nucleoside present in Escherichia coli tRNA, was recently proposed to be the major chromophore leading to near UV (315-400 nm) induced growth delay. This is established by the isolation of mutants exhibiting a reduced growth delay. The selection procedure involves several successive cycles of 365-nm illumination of the cells in the stationary phase, followed by growth for 2 or 3 generations. After the 8th cycle, the level of 4-thiouridine in the culture decreases to 20% of the original level and all individual clones tested show a 4-thiouridine deficiency. One mutant exhibiting a complete lack of 4-thiouridine in its tRNA was characterized. In the dark the growth characteristics of the mutant and of the parental strain are indistinguishable. In contrast after near-UV illumination the nuv mutation abolishes the growth delay and considerably reduces the photoprotection efficiency.