Thirteen different mutants of E coli K12 selected for a reduced near ultra-violet induced growth delay have been isolated. The t-RNAs extracted from these clones have all a depressed content of 4-thiouridine. One of these mutants, called Nop has an almost negligible growth delay and completely lacks 4-thiouridine in its t-RNAs. Thus we provide genetic proofs that 4-thiouridine is the chromophore for growth delay.