Genetic analysis of higher plant mitochondria has been limited by a scarcity of defined mutations with known progenitors. The maternally-inherited Nonchromosomal stripe mutants of maize, NCS2 and NCS3, have drastic effects on growth, development and yield. Both mutations are lethal at one or more stages of the life cycle and mutant plants appear to contain a mixture of defective and normal organelles. Specific alterations have been found in the mitochondrial DNA of each of these independent, phenotypically distinguishable mutants. Each also has a different effect on mitochondrial function: NCS2 affected plants have much reduced amounts of a single polypeptide and NCS3 affected plants have a defect which interferes with mitochondrial translation. We propose to characterize the genetic basis of the alterations in the mitochondrial genomes of NCS2 and NCS3, to identify the products of the mutated loci and to define the altered transcription units. We will further characterize observed ultrastructural alterations in mitochondria and chloroplasts and ascertain in the NCS2 mutation interrupts a developmentally regulated maize mitochondrial gene.