Mitochondrial genes are found on minicircle DNA molecules in the mesozoan animal Dicyema

Animal mitochondrial DNA genomes are generally single circular molecules, 1 4-20 kb in size, containing a number of functional RNAs and 13 protein-codi ng genes. Among these, the COI, COII and COIII genes encode three subunits of cytochrome c oxidase. We have isolated and characterized these three mit ochondrial genes from the mesozoan Dicyema, a primitive multicellular anima l. Surprisingly, the COI, COII and COIII genes are encoded on three small, separate circular DNA molecules (minicircles) of length 1700, 1599 and 1697 bp, respectively. We estimated the copy number of each minicircle at 100 t o 1000 per cell, and have shown a mitochondrial localization of the minicir cles by in situ hybridization. Furthermore, we could not detect a putative "maxicircle" DNA molecule containing any combination of the COI, COII and C OIII genes using either PCR or genomic Southern hybridization. Thus, our re sults show a novel mitochondrial genome organization in the mesozoan animal Dicyema. (C) 1999 Academic Press.