South African toad

RNA polymerase III promoters differ significantly from RNA polymerase II promoters in that they are located downstream from the transcription start site and within the transcribed segment of the DNA. For example, in the 5S RNA gene of the South African toad Xenopus laevis) the promoter is between 45 and 95 nucleotides downstream from the start point. Thus, the binding sites on RNA polymerase III are reversed with respect to the transcription direction, as compared with RNA polymerase II. That is, RNA polymerase II reaches forward to find the start point, and RNA polymerase III reaches backward. In fact, RNA polymerases can slide in either direction along a DNA template however, they can only synthesize RNA molecules in a 5 3 direction. 

Xenopus laevis, the South African clawed toad, is a frog, not a toad. It is often used as a model organism for the study of early vertebrate development. 

In vitro studies with abdominal skin of Rana pipiens demonstrated that naphthalene inhibited sodium transport after exposure to 4.4mg/L for 30 min. Some data were available on biological effects of benzo[a]pyrene, 3-methylcholanthrene, and perylene to reptiles and amphibians. Implantation of 1.5 mg of benzo[a]pyrene crystals into the abdominal cavity of adult South African clawed toads (Xenopus laevis) produced lymphosarcomas in 11 of the 13 toads (85%) ter 86-288 days. Immature toads were more resistant with only 45% bearing lymphoid tumors of liver, kidney, spleen, or abdominal muscle 272-310 days after implantation of 1.5 mg of benzo[a]pyrene crystals in the dorsal lymph sac or abdominal cavity. Implantation of 3-methylcholanthrene crystals into X. laevis provokes development of lymphoid tumors similar to those occurring naturally in this species moreover, these... 

Brown, D. D., and Littna, E. (1964). RNA synthesis during the development of Xenopus laevis, the South African Clawed Toad. J. Mol. Biol. 8, 669-687. 

---