Expression of the linear DNA plasmid pRS64 in the plant pathogenic fungus Rhizoctonia solani

Expression and localization of the linear DNA plasmid-encoded protein (RS224) in Rhizoctonia solani AG2-2

Expression of the linear DNA plasmid-encoded protein (RS224) from the plant-pathogenic fungus Rhizoctonia solani isolate H-16, anastomosis group 2-2, and its localization were studied. Extracts from Escherichia coli cells expressing the open reading frame (ORF) of RS224 (RS224ORF in pRS224) contain a 92-kDa T7.Tag-RS224orf fusion protein. Antisera raised against the fusion protein obtained from E. coli cells cross-reacted with a 90-kDa protein in the mycelia. To analyze the subcellular localization of the 92-kDa protein, mycelia of R. solani were disrupted and fractionated. Antibodies against RS224 proteins specifically reacted to the mitochondrial fraction, suggesting that RS224 is localized in mitochondria.

Natural plasmids of filamentous fungi

Among eukaryotes, plasmids have been found in fungi and plants but not in animals. Most plasmids are mitochondrial. In filamentous fungi, plasmids are commonly encountered in isolates from natural populations. Individual populations may show a predominance of one type, but some plasmids have a global distribution, often crossing species boundaries. Surveys have shown that strains can contain more than one type of plasmid and that different types appear to be distributed independently. In crosses, plasmids are generally inherited maternally. Horizontal transmission is by cell contact. Circular plasmids are common only in Neurospora spp., but linear plasmids have been found in many fungi. Circular plasmids have one open reading frame (ORF) coding for a DNA polymerase or a reverse transcriptase. Linear plasmids generally have two ORFs, coding for presumptive DNA and RNA polymerases with amino acid motifs showing homology to viral polymerases. Plasmids often attain a high copy number, in excess of that of mitochondrial DNA. Linear plasmids have a protein attached to their 5' end, and this is presumed to act as a replication primer. Most plasmids are neutral passengers, but several linear plasmids integrate into mitochondrial DNA, causing death of the host culture. Inferred amino acid sequences of linear plasmid ORFs have been used to plot phylogenetic trees, which show a fair concordance with conventional trees. The circular Neurospora plasmids have replication systems that seem to be evolutionary intermediates between the RNA and the DNA worlds.