Involvement of S2 episomal sequences in the generation of NCS4 deletion mutation in maize mitochondria

Cytoplasmic male sterility (CMS) and the abnormal-growth, nochromosomal stripe (NCS) phenotypes are cytoplasmically determined traits in plants. Mitochondrial DNA rearrangements involving short repeats appear to be responsible for the production of CMS reversions to fertility and NCS mutations. NCS4, a new mutant of maize CMS-S, exhibits both abnormal growth and male fertility. This mutant is unique because both mutations occurred within the same plant. Free S1 and S2 episomes normally found in CMS-S mitochondria have been lost from NCS4 plants. An S2 sequence has recombined aberrantly with a ribosomal protein coding region, rps3/rpl16. One end of the S2 sequence and the 5' end of the rps3/rpl16 transcription unit are absent from the NCS4 recombinant genome. Loss of mitochondrial ribosomal protein function is lethal; therefore, NCS4 plants are heteroplasmic for the rps3 deletion. Loss of S sequences from CMS-S mitochondria is not lethal and plants regain pollen function. Thus, although NCS4 plants have very abnormal plant phenotypes, they are male-fertile.