Molecular analyses of the NCS2 mutant

--Deborah L. Roussell and Kathleen J. Newton

The NCS2 phenotype is characterized by reduced growth and vigor, sectors of pale green tissue on the leaf and sectors of small kernels on the ear. Our previous studies have demonstrated an absolute correlation between the mutant phenotype and a specific mitochondrial DNA rearrangement (Newton and Coe,PNAS 82:6879-83, 1986). Additionally, one mitochondrial protein is produced at reduced levels during in organellar protein synthesis assays (Feiler and Newton, EMBO J. 6:1535-39, 1987). The NCS2- defined DNA sequences are transcribed in both the mutant and progenitor mitochondrial genomes. There are at least three distinct transcription units within this region, one of which is characterized by multiple RNAs. In the mutant mitochondria, however, this transcription pattern is altered; large molecular-weight transcripts accumulate. We are currently examining whether these multiple transcripts result from multiple initiation or processing events and whether these processes have been disrupted in mutant mitochondria.

Because the NCS2 phenotype is distinguished by pale green stripes on the leaf, we predicted that the mitochondrial DNA rearrangement and resulting loss of mitochondrial function must exert a pleiotropic effect on the chloroplast. Indeed, electron micrographs of the pale green leaf tissue reveal that both the mitochondria and chloroplasts have markedly reduced inner membrane and thylakoid membrane structures respectively (Thompson and Newton, MNL 61:46, 1987). Further analyses (low temperature fluorescence spectra, fluorescence induction kinetics, and PAGE of thylakoid proteins) suggest that the chloroplasts isolated from pale-green sectors are functionally as well as morphologically distinct from those in dark-green sectors.

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