The illumination of Escherichia coli cells with UVA light, 320 nm less than or equal to lambda less than or equal to 380 nm, triggers a transient growth and division delay. The built-in 4-thiouridine chromophore which absorbs light at 340 nm leads to the quantitative 8-13 crosslinking of a number of tRNA species corresponding to 50% of the bulk tRNA molecules. Determination of the tRNA acylation level by the various aminoacids shows that only the tRNA species acylated by Phe and Pro are strikingly affected in vivo. Both acylation levels decrease to less than 10% of their initial value during the illumination period, remain stable all along the growth lag and increase concomitantly with cell mass when growth resumes. Hence tRNA(Phe) and tRNA(Pro) are the UVA light molecular targets triggering growth delay and related effects of biological significance such as cell volume reduction, photoprotection and protection against UV mutagenesis (antiphotomutagenesis).
