Characterization of a novel plasmid-like element in Neurospora crassa derived mostly from the mitochondrial DNA

We have identified a plasmid-like element within mitochondria of Neurospora crassa strain stp-B1. It is derived from the EcoRI-4 and EcoRI-6 regions of the mitochondrial DNA, and an additional 124 bp DNA segment of unknown origin. The plasmid DNA consists of an oligomeric series of circular molecules of monomer length 2.2 kbp. The abundance of the plasmid suggests its autonomous replication and the presence of an efficient origin of replication. An unusually large number of palindromes capable of forming secondary structures are present in the plasmid. Such a palindrome, located near sequences reminiscent of mammalian and fungal mtDNA origins of replication, may define the replication origin of the plasmid. This putative origin might also represent the replication origin of the wild-type mtDNA.