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

Illumination with near-ultraviolet light triggers synthesis of ppGpp (guanosine 3'-diphosphate 5'-diphosphate) not only in growing Escherichia coli cells containing the putative chromophore 4-thiouridine in their tRNAs [Ramabhadran, T. V and Jagger, J. (1976) Proc. Natl Acad. Sci. USA, 73, 59--69], but also in nuv- cells which lack 4-thiouridine. The burst of ppGpp in nuv- cells is, however, induced exclusively by light of wavelengths shorter than 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 tRNAs. A candidate likely to trigger this effect is a 5-methylaminomethyl-2-thiouracil residues present in the first position of the anticodon loop of tRNAGlu, tRNALys and one tRNAGln isoacceptor. In conditions in vitro, this base is highly photoreactive at wavelengths shorter than 350 nm. Furthermore, near-ultraviolet-photomodified tRNAGlu and tRNALys become poor substrates of their acylation enzyme.