The tobacco mitochondrial ATPase subunit 9 gene is closely linked to an open reading frame for a ribosomal protein.

A transcribed segment of mitochondrial DNA (mtDNA) from Nicotiana tabacum contains the F0-ATPase subunit 9 gene, an open reading frame with homology to the E. coli small subunit ribosomal protein S13 and an open reading frame with homology to a portion of the mammalian "URF 1" protein, recently shown to be a component of the NADH:ubiquinone reductase complex (NADH:Q 1). The transcriptional patterns of the tobacco ATPase 9 gene and S13-like open reading frame share eight RNA species indicating the two sequences are part of the same transcriptional unit. A maize mtDNA fragment contains the S13 homologous sequence and the NADH:Q 1 homologous sequence in an orientation similar to tobacco. The S13-like sequence is present as a single copy in maize and tobacco, as two copies in wheat, and is absent in pea and bean. We discuss the distribution and orientation of the S13-like and "URF 1"-like sequences and the possibility that they are active genes.