Structure of the telomeric ends of mt DNA, transcriptional analysis and complex I assembly in the dum24 mitochondrial mutant of Chlamydomonas reinhardtii.

The dum24 mutant of Chlamydomonas reinhardtii contains four types of altered mitochondrial linear genomes: two types of deleted monomers and two types of dimers resulting from fusions between some monomers via their deleted ends. All molecules lack at least cob, nd4 and the 3' end of nd5, three adjacent genes located in the left part of the genome. We present evidence showing that in dum24, as in other deletion mutants, the deletions extend to the left telomeric end, and propose that the only replicative forms in the mutants are the dimeric DNA molecules that possess intact telomeric structures at both ends. Two abnormally large transcripts produced from chimeric genes are detected in dum24, which throws some light on the location of potential promoter sequences and processing signals in the mitochondrial genome. Using BN-PAGE analysis and immunological methods to detect complex I, we further show that dum24 mitochondria do not possess the normal multimeric complex I (850 kDa), but produce a smaller, partially assembled, complex (650 kDa), demonstrating a role for ND4 and/or ND5 subunits(s) in complex I assembly.