Genes nested opposite the coding sequences of their host genes are very rare, and have been observed in prokaryotes, and more recently, in yeast (S. cerevisiae) and in Tetrahymena thermophila.
The first ribozymes were discovered in the 1980s by Thomas R. Cech, who was studying RNA splicing in the ciliated protozoan Tetrahymena thermophila and Sidney Altman, who was working on the bacterial RNase P complex.