Characterization of two cDNA clones encoding isozymes of the F(1)F(0)-ATPase inhibitor protein of rice mitochondria.

Two cDNA clones encoding F(1)F(0)-ATPase inhibitor proteins, which are loosely associated with the F(1) part of the mitochondrial F(1)F(0)-ATPase, were characterized from rice (Oryza sativa L. cv. Nipponbare). A Northern hybridization showed that the two genes (designated as IF(1)-1 and IF(1)-2) are transcribed in all the organs examined. However, the steady-state mRNA levels varied among organs. A comparison of the deduced amino acid sequences of the two IF(1) genes and the amino acid sequence of the mature IF(1) protein from potato revealed that IF(1)-1 and IF(1)-2 have N-terminal extensions with features that are characteristic of a mitochondrial targeting signal. To determine the subcellular localization of the gene products, the IF(1)-1 or IF(1)-2 proteins were fused in frame to the green fluorescent protein (GFP) or the fused GFP-beta-glucuronidase, and expressed transiently in onion or dayflower epidermal cells. Localized fluorescence was detected in mitochondria, confirming that the two IF(1) proteins are targeted to mitochondria.