Molecular basis of the ‘ box effect’

In the mitochondrial DNA of Saccharoimyces cerevisiae , the genes cob‐box and oxi3 , coding for apocytochrome b and cytochrome oxidase subunit I respectively, are split. Several mutations located in the introns of the cob‐box gene s of cytochrome b and cytochrome oxidase subunit I (this is known as the ‘ box effect’).‐We have elucidated the molecular basis of this phenomenon: these mutants are unable to excise the fourth intron of oxi3 from the cytochrome oxidase subunit I pre‐mRNA; the absence of a functional bI4 mRNA maturase, a trans ‐acting factor encoded by the fourth intron of the cob‐box gene explains this phenomenon. This maturase was already known to control the excision of the bI4 intron; consequently we have demonstrated that it is necessary for the processing of two introns located in two different genes. Mutations altering this maturase can be corrected, but only partially, by extragenic suppressors located in the mitochondrial ( mim2 ) or in the nuclear ( NAM2 ) genome. The gene product of these two suppressors should, therefore, control (directly or indirectly) the excision of the two introns as the bI4 mRNA maturase normally does.