Chromosome Associated Plasmid

To gain information on the chromosomal DNA of isolates belonging to AG4 of R. solani, we used the orthogonal-field-alter nation gel electrophoresis (OFAGE) technique to separate large DNAs from the filamentous fungus. At least six chromosome-sized DNA bands from isolates RI-64 and 1271 of R. solani were separated using OFAGE. Thus, the total genome size ofR. solani is estimated to be more than 11 Mb. The assignment of linkage groups to the resolved chromosomal DNA bands might be accomplished using linkage group-specific probes.

Fungal plasmids share homology with either nuclear DNA (Wright and Cummings 1983) or mitochondrial DNA, or both (Van den Boogert et al. 1982; Samac and Leong 1989). Southern hybridization analysis with nearly full-length clones of pRS64-1 and -2 (excluding the terminal full regions) identified a homologous sequence within a small chromosome-sized DNA band of approximately 1.1 Mb. Sequence homology to pRS64 was also maintained within the chromosomal DNA of isolate 1271 of AG4, which does not possess the plasmid. The plasmid showed no homology to the mitochondrial DNA of isolate 1271 (Wako et al. 1991). This finding suggests that pRS64 are stably integrated into the chromosomal DNA. Study of the plasmid sequence will help to identify their integrated forms in the fungal DNA and reveal how integration over the eons has impacted the evolution of the mitochondrial and nuclear genetics of Rhizoctonia.

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