The kalilo linear senescence-inducing plasmid of Neurospora is an invertron and encodes DNA and RNA polymerases

Abstract

The nucleotide sequence of kalilo, a linear plasmid that induces senescence in Neurospora by integrating into the mitochondrial chromosome, reveals structural and genetic features germane to the unique properties of this element. Prominent features include: (1) very long perfect terminal inverted repeats of nucleotide sequences which are devoid of obvious genetic functions, but are unusually GC-rich near both ends of the linear DNA; (2) small imperfect palindromes that are situated at the termini of the plasmid and are cognate with the active sites for plasmid integration into mtDNA; (3) two large, non-overlapping open-reading frames, ORF-1 and ORF-2, which are located on opposite strands of the plasmid and potentially encode RNA and DNA polymerases, respectively, and (4) a set of imperfect palindromes that coincide with similar structures that have been detected at more or less identical locations in the nucleotide sequences of other linear mitochondrial plasmids. The nucleotide sequence does not reveal a distinct gene that codes for the protein that is attached to the ends of the plasmid. However, a 335-amino acid, cryptic, N-terminal domain of the putative DNA polymerase might function as the terminal protein. Although the plasmid has been co-purified with nuclei and mitochondria, its nucleotide composition and codon usage indicate that it is a mitochondrial genetic element.

Strikingly Bacteria-Like and Gene-Rich Mitochondrial Genomes throughout Jakobid Protists

“…S4 E , Supplementary Material online), suggesting unidirectional replication that starts at both ends and terminates in the middle of the genome. This situation is reminiscent of linear-plasmid (invertron) replication in Neurospora (Chan et al 1991) and is consistent with a plasmid insertion event as postulated above, transforming the mitochondrial genome architecture from circular to linear. Genome linearization mediated by plasmid insertion has been previously demonstrated in maize mitochondria, where the distal sequences of the linearized mtDNA originate from a plasmid (Schardl et al 1984).…”

The mitochondrial genome of the pathogenic yeast Candida subhashii: GC-rich linear DNA with a protein covalently attached to the 5′ termini

“…However, in fungal cytoplasmic and mitochondrial linear plasmids, the TP is generated from the N-terminal domain of the plasmid-encoded DNA polymerase (Kim et al , 2000; Takeda et al , 1996), or an entire DNA polymerase molecule may serve as a TP (Chan et al , 1991; Vierula et al , 1990). At present, we cannot rule out the possibility that in C. subhashii mitochondria, the TP is encoded by an ORF of unknown function, such as orf756 .…”

Mitochondrial Genome Rearrangements in Glomus Species Triggered by Homologous Recombination between Distinct mtDNA Haplotypes

“…Plasmid structure varies widely, from circular molecules to linear invertrons with terminal inverted repeats (TIRs). Both types harbor one or two open reading frames (ORFs) encoding DNA and/or RNA polymerase genes (Nargang et al 1984; Akins et al 1988; Pande et al 1989; Sakaguchi 1990; Shiu-Shing Chan et al 1991; Court and Bertrand 1992; Li and Nargang 1993). Plasmids have also been shown to integrate into fungal mtDNA through recombination and can either be cryptic or elicit changes in phenotypic expression (Akins et al 1986; Robison et al 1991; Griffiths 1992; Hänfler et al 1992; Oeser et al 1993; Hermanns et al 1994).…”