Abstract: Distinct metabolic pathways can intersect in ways that allow hierarchical or reciprocal regulation. In a screen of respiratory-deficient yeast gene deletion strains for defects in mitochondrial RNA processing, we found that lack of any enzyme in the mitochondrial fatty acid biosynthetic pathway (FAS II) led to inefficient 5' processing of mitochondrial precursor tRNAs by RNase P. In particular, the precursor containing both the RNase P RNA (RPM1) and tRNA(pro) accumulated dramatically. Subsequent Pet127-driven 5' processing of RPM1 was blocked. The FAS II pathway defects resulted in loss of lipoic acid attachment to subunits of three key mitochondrial enzymes, which suggests that octanoic acid produced by the pathway is the sole precursor for lipoic acid synthesis and attachment. The protein component of yeast mitochondrial RNase P, Rpm2, is not modified by lipoic acid in the wild-type strain, and it is imported in FAS II mutant strains. Thus, a product of the FAS II pathway is required for RNase P RNA maturation, which positively affects RNase P activity. In addition, a product is required for lipoic acid production, which is needed for the activity of pyruvate dehydrogenase, which feeds acetyl-CoA into the FAS II pathway. These two positive feedback cycles may provide switch-like control of mitochondrial gene expression in response to the metabolic state of the cell.