Conversion of a mitochondrial gene for mammalian cytochrome c oxidase subunit II into its universal codon equivalent and expression in vivo and in vitro

Abstract

To begin to assess the independent structural and functional characteristics of the mitochondrially encoded subunits of mammalian cytochrome c oxidase, we have converted the cloned mitochondrial gene for rat subunit II (coxII) into its universal codon equivalent (ucoxII) by oligonucleotide-directed, site-specific mutagenesis. This involved synthesizing 12 oligodeoxynucleotides to achieve the 13 ATA to ATG and the 5 TGA to TGG changes needed. To express ucoxII in Escherichia coli, we used a number of different expression vectors in which the promoters and ribosome-binding sequences of the messenger RNA were varied. While ucoxII alone was expressed at a low level, a striking increase in the level of expression resulted when the ucoxII gene was fused to other E. coli genes. The COXII peptide was identified by proteolytic digestion, partial sequencing, and reaction with specific antisera. A cro-beta-galactosidase-COXII fusion protein has been purified, characterized, and used to produce polyclonal antibodies to the COXII peptide. The ucoxII gene was also expressed in a cell-free translation system and in Xenopus oocytes, yielding a nondenatured, membrane-associated peptide with the same apparent molecular weight as authentic subunit II. In oocytes and in a reticulocyte lysate in vitro system supplemented with microsomal membranes, the protein is glycosylated and coisolates with the washed membrane fraction. In both cases, the COXII peptide is soluble under mild conditions in a nonionic detergent and is precipitable by antibodies to subunit II. The production of subunit II in the in vitro translation system is stimulated as strongly by addition of soybean phospholipid vesicles as by microsomal membranes, providing further evidence of membrane insertion and stabilization.(ABSTRACT TRUNCATED AT 250 WORDS)