Abstract
Treatment of the ciliated protozoan Tetrahymena thermophila with 8-methoxypsoralen combined with long wavelength ultraviolet irradiation (UVA, λ ˜ 360 nm) resulted in a dose dependent equal inhibition of the synthesis of rRNA, 5.8S, 5S and tRNA. Similar results were obtained with 3-carbethoxy-8-methylpsoralen which predominantly forms DNA mono-adducts. In contrast the synthesis of tRNA in T. thermophila was much less sensitive than that of rRNA, 5.8S and 5S RNA to treatment with short wavelength ultraviolet irradiation (UVB, λ ˜ 254 nm). These results are interpreted in favor of a mechanism by which psoralen-DNA adducts (crosslinks ≫ monoadducts) inhibit RNA transcription initiation (in contrast to UVB which causes premature chain termination). Furthermore it is argued that RNA synthesis is regulated in equally sized domains regardless of the gene-size.