CLONING AND CHARACTERIZATION OF KICOX18, A GENE REQUIRED FOR ACTIVITYOF CYTOCHROME-OXIDASE IN KLUYVEROMYCES-LACTIS

We describe the isolation and initial characterization of KlCOX18, a g ene that is essential for the assembly of a functional cytochrome oxid ase in the yeast Kluyveromyces lactis. Cells carrying a recessive nucl ear mutation in this gene are respiratory deficient and contain reduce d levels of cytochromes a and a(3). The KlCOX18 gene has been cloned b y complementation of the respective nuclear mutation, sequenced, and d isrupted. KlCOX18 is located on chromosome II and contains an open rea ding frame of 939 base pairs. The corresponding protein exhibits 70.4% similarity to the Cox18p of Saccharomyces cerevisiae. It contains thr ee possible membrane-spanning domains and a putative amino-terminal mi tochondrial import sequence. The strain carrying a null mutation in Kl COX18 does not grow on non-fermentable carbon sources and is deficient in both cytochrome c oxidase and respiratory activity. It is proposed that KlCox18p, like its S. cerevisiae counterpart, provides an import ant function at a later step of the cytochrome oxidase assembly pathwa y.