Expression of ORF1 of the linear 2.3 kb plasmid of maize mitochondria: product localization and similarities to the 130 kDa protein encoded by the S2 episome.

ORF1 of the ubiquitous 2.3 kb linear plasmid of maize mitochondria encodes a 39 kDa protein detected with polyclonal antibodies raised to a beta-galactosidase:ORF1 fusion protein. Almost half of this protein is similar to a domain of the 130 kDa protein encoded by the S2 episome of mitochondria from cytoplasmic male-sterile lines. Antisera raised to the ORF1 2.3 kb plasmid product cross-reacts with the ORF1, 130 kDa protein from the S2 episome. Despite the shared domain, the proteins are differentially localized: the 130 kDa protein is membrane-associated while the ORF1 protein is found in the matrix. We discuss possible functions of the ORF1 protein.