Abstract: Three missense SURF1 mutations identified in patients with Leigh syndrome were evaluated in the yeast homolog Shy1 protein. Introduction of two of the Leigh mutations, F(249)T and Y(344)D, in Shy1 failed to significantly attenuate the function of Shy1 in CcO biogenesis as seen with the human mutants. In contrast, a G(137)E substitution in Shy1 results in a non-functional protein conferring a cytochrome oxidase (CcO) deficiency. The G(137)E Shy1 mutant phenocopied shy1Delta cells in impaired Cox1 hemylation and low mitochondrial copper. A genetic screen for allele-specific suppressors of the G(137)E mutant Shy1 revealed Coa2, Cox10 and a novel factor designated Coa4. Coa2 and Cox10 are previously characterized CcO assembly factors. Coa4 is a twin Cx9C motif mitochondrial protein localized in the intermembrane space and associated with the inner membrane. Cells lacking Coa4 are depressed in CcO activity but show no impairment in Cox1 maturation or formation of the Shy1-stabilized Cox1 assembly intermediate. To glean insights into the functional role of Coa4 in CcO biogenesis, an unbiased suppressor screen of coa4Delta cells was conducted. Respiratory function of coa4Delta cells was restored by the overexpression of CYC1 encoding cytochrome c. Cyc1 is known to be important at an ill-defined step in the assembly and/or stability of CcO. This new link to Coa4 may begin to further elucidate the role of Cyc1 in CcO biogenesis.