Studies of yeast, algae and plants have provided genetic and biochemical evidence that the splicing reaction of organellar localized group II introns either depends on proteins encoded by the introns themselves ('maturases') or encoded by other genes of the host organisms. However, only a few of those proteins have been identified to date and characterized in more detail. In order to find new nuclear encoded proteins that assist group II splicing, we screened a complete knockout library of Saccharomyces cerevisiae strain BY4741 consisting of 4878 viable haploid clones. The strain contains a rho+ mitochondrial genome with a set of 13 introns including the three group II introns (aI1, aI2, aI5gamma) in the gene encoding cytochrome-c-oxidase subunit 1 (COX1) and the single group II intron (bI1) in the gene encoding cytochrome b (CYTB). In our screen and initial molecular analysis, we focus on intron aI5gamma, the last intron in the COX1 gene.