We have previously described the characterisation of an abundant mitochondrial protein (p40) that binds specifically to 5'-untranslated leaders of mitochondrial mRNAs in yeast. p40 consists of two polypeptides with M(r) of 40 and 39 kDa. Limited sequence analysis of p40 identifies it as the Krebs cycle enzyme NAD(+)-dependent isocitrate dehydrogenase (Idh). Both enzyme and RNA-binding activities are specifically lost in cells containing disruptions in either IDH1 or IDH2, the nuclear genes encoding the two subunits of the enzyme, thus conclusively identifying p40 as Idh and showing that both activities are dependent on the simultaneous presence of both subunits. Although we still must ascertain whether and how either function of Idh is regulated and whether the two functions are compatible or mutually exclusive, this combination of dehydrogenase activity and RNA-binding in a single protein may be part of a general regulatory circuit linking the need for mitochondrial function to mitochondrial biogenesis.