Electron microscopic analysis supports a dual role for the mitochondrial telomere-binding protein of Candida parapsilosis

Linear mitochondrial genomes exist in several yeast species which are close ly related to yeast that harbor circular mitochondrial genomes. Several lin es of evidence suggest that the conversion from one form to another occurre d accidentally through a relatively simple mechanism. Previously, we (L.T. & J.N.) reported the identification of the first mitochondrial telomere-bin ding protein (mtTBP) that specifically binds a sequence derived from the ex treme end of Candida parapsilosis linear mtDNA, and sequence analysis of th e corresponding nuclear gene MTP1 revealed that mtTBP shares homology with several bacterial and mitochondrial single-stranded (ss) DNA-binding (SSB) proteins. In this study, the DNA-binding properties of mtTBP in vitro and i n vivo were analyzed by electron microscopy (EM). When M13 ssDNA was used a s a substrate, mtTBP exhibited similar DNA binding characteristics as human mitochondrial SSB: mtTBP formed protein globules along the DNA substrate, and the bound proteins were randomly distributed, indicating that the bindi ng of mtTBP to M13 ssDNA is not highly cooperative. EM analysis demonstrate d that mtTBP is able to recognize the 5' single-stranded telomeric overhang s in their natural context. Using isopycnic centrifugation of mitochondrial lysates of C. papsilosis we show that mtTBP is a structural part of mitoch ondrial nucleoids of C. parapsilosis and is predominantly bound to the mito chondrial telomeres. These data support a dual role of mtTBP in mitochondri a of C. parapsilosis, serving both as a typical mitochondrial SSB and as a specific component of the mitochondrial telomeric chromatin. (C) 2001 Acade mic Press.