TRANSFER RNA-PHE AND TRANSFER RNA-PRO ARE THE NEAR-ULTRAVIOLET MOLECULAR TARGETS TRIGGERING THE GROWTH DELAY EFFECT

The illumination of Escherichia coli cells with UVA light, 320 .ltoreq. .lambda. .ltoreq. 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 amino acids showed that only the tRNA species acylated by Phe and Pho 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 tRNAPhe and tRNAPro are the UVA light molecular targets tiggering growth delay and related effects of biological significance such as cell volume reduction, photoprotection and protection against UV mutagenesis (antiphotomutagenesis).