Mitochondrial Location of DNA Plasmids

Most of the linear DNA plasmids of filamentous fungi are thought to reside within mitochondria (Samac and Leong 1989). To investigate the location of DNA plasmids of R. solani AG4 and AG2-2, total DNAs or DNA isolated from intact mitochondria were separated by CsCl-bisbenzimide centrifuga-tion. Plasmid bands were detected only in the mitochondrial DNA fractions. The plasmids were resistant to DNase treatment of intact mitochondria, indicating that they were located in the mitochondria. DNA plasmids (pRS64 and pRS224) from R. solani are associated with mitochondria (Chen et al. 1992; Kuradate et al. 1996; Wako et al. 1991). Jabaji-Hare et al. (1994) also demonstrated that DNA plasmids (pRS104 and pRS188) from R. solani are associated with mitochondria in cellular fractionation experiments.

Mitochondrial DNAs fractionated on CsCl-bisbenzimide density gradients were examined. Linear plasmids were found in the mitochondria in all examined isolates of F. oxysporum f.sp. conglutinans, f.sp. raphani, f.sp. matthioli, f.sp. arctii, f.sp. melongenae, f.sp. lini, and f.sp. batatas from a global collection (Hirota et al. 1992; Kistler et al. 1987; Kistler and Leong 1986). Our preliminary work indicated that the plasmids were resistant to DNase treatment of intact mitochondria, indicating their mitochondrial location.

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