Identification of a maize nuclear gene which influences the size and number of cox2 transcripts in mitochondria of perennial teosintes

Abstract

The involvement of nuclear genes in mitochondrial gene expression was investigated by identifying alterations in mitochondrial gene expression that occur when teosinte cytoplasms are introduced into certain maize inbred nuclear backgrounds. The cytoplasms from the teosintes Zea perennis, Zea diploperennis, and Zea luxurians were introduced into the maize A619 or W23 lines by recurrent backcrossing. Northern analysis revealed that the Z. perennis and Z. diploperennis mitochondrial cox2 transcript patterns were dependent upon the maize nuclear genotype. In a W23 nuclear background, these teosinte mitochondria have two major transcripts of 1.9 and 1.7 kb, whereas in an A619 background, they have three major transcripts of 1.9, 1.5 and 1.3 kb. No effect of nuclear background on cox2 transcripts was observed for plants possessing Z. luxurians cytoplasm. All teosinte-maize combinations possess larger, minor cox2 transcripts of 3.9, 3.3 and 3.0 kb; nuclear background has no effect on these transcripts. Immunoblot analysis showed a threefold reduction of the COXII polypeptide in Z. perennis-A619 combinations compared to Z. perennis-W23 combinations. All the major and minor transcripts possess both cox2 exons. The cox2 intron is missing from all the major transcripts and is present only in the 3.9- and 3.0-kb minor transcripts. The 1.7-and 1.3-kb transcripts are missing untranslated regions 3' to the cox2 gene; therefore at least some of the size heterogeneity is due to differential termination or downstream processing. Genetic analyses indicate that a single nuclear gene is responsible for the observed differences in the major cox2 transcripts, and that A619 carries the dominant allele. This gene, designated Mct, is specific for cox2, as no transcript size differences were observed for the other two mitochondrial cox genes.