In fungal mitochondria (also in chloroplasts), introns of group I and group II are known, which are involved in the synthesis of a protein, RNA maturase. This maturase causes the splicing of the intron carrying the coding sequence of the same maturase. The mutations in these introns are defective in splicing and are trans-acting in contrast to cw-acting mutations involved in other splicing mechanisms discussed earlier. An example of this splicing mechanism is found in the mitochondrial gene for cytochrome-b, also called box gene. It has been shown that successive splicing in mRNA leads to the production of overlapping proteins. In other words an intermediate mRNA molecule has an exon sequence (144 codons), which is read with a part of the intron (840 bases = 280 codons) as an open reading frame, to give rise to the protein RNA maturase (the remainder of intron is not translated). This open reading frame is actually generated by splicing of the first intron. This maturase is believed to be responsible for splicing out the second intron, thus producing the mRNA, which will then disrupt the reading frame for maturase and will be translated into cytochrome b. This mechanism is illustrated in Figure 33.8. It is not necessary that the maturase has a catalytic activity, but may only recognize the splicing site, so that the splicing is achieved by a complex consisting of maturase, and other proteins and/or RNA molecules.