HAIRPIN AND DIMER STRUCTURES OF LINEAR PLASMID LIKE DNAS IN MITOCHONDRIA OF PARAMECIUM-CAUDATUM

The molecular structure of plasmid-like DNAs (designated type-II) whic h were isolated from mitochondria in the ciliated protozoan Paramecium caudatum was characterized. These type-II DNAs are always detected as a set of four kinds with sizes of 8.2, 4.1, 2.8 and 1.4 kb. The DNAs of 8.2 and 2.8 kb exist as dimers consisting of 4.1- and 1.4-kb monome r molecules, respectively. Electron microscopic observations indicated configurations of a hairpin structure that had a protruding end of si ngle-stranded DNA in one terminus and a loop in the other terminus. Th e monomers stick together with base-pairing in opposite directions at the protruding end to form the dimers, suggesting the presence of inve rted repeats. These unusual dimers may have a role in replication of t he DNAs in which the monomers can serve as a primer for each other.