tRNA Thiolated Pyrimidines as Targets for Near‐Ultraviolet‐Induced Synthesis of Guanosine Tetraphosphate in Escherichia coli

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Abstract
Illumination with near-UV light triggers synthesis of ppGpp (guanosine 3''-diphosphate 5''-diphosphate) in growing E. coli cells containing the putative chromophore 4-thiouridine in their tRNA and in nuv- cells which lack 4-thiouridine. The burst of ppGpp in nuv- cells is induced exclusively by light of wavelengths < 350 nm. Its maximum level is half that obtained in the parental strain. This ppGpp synthesis is also under the control of the relA gene, indicating that it is due to the accumulation of uncharged tRNA. A candidate likely to trigger this effect is a 5-methylaminomethyl-2-thiouracil residues present in the 1st position of the anticodon loop of tRNAGlu, tRNALys and 1 tRNAGln, isoacceptor. In conditions in vitro, this base is highly photoreactive at wavelengths > 350 nm. Near-UV-photomodified tRNAGlu and tRNALys become poor substrates of their acylation enzyme.