The transcriptional activator HAP4 is a high copy suppressor of an oxa1 yeast mutation.

Oxa1p is a key component of the machinery for the insertion of membrane proteins in mitochondria, and in the yeast Saccharomyces cerevisiae, the deletion of OXA1 impairs the biogenesis of the three respiratory complexes of dual genetic origin. Oxa1p is formed from three domains located in the intermembrane space, the inner membrane and the mitochondrial matrix. We have isolated a high copy suppressor able to partially compensate for the respiratory deficiency caused by a large deletion of the matrix domain. We show that the suppressor gene corresponds to the nuclear transcriptional activator Hap4p which is known to regulate respiratory functions.