A 7.5-kbp region of the maize (T cytoplasm) mitochondrial genome contains a chloroplast-like trnI (CAT) pseudo gene and many short segments homologous to chloroplast and other known genes.

The DNA sequence of a 7.5-kbp region from the maize cmsT mt genome was determined. Nucleotide-sequence analysis of this region reveals the presence of a ct-like trnI (CAT) gene, orf31 and orf48 which are respectively homologous to the trnI (CAT) gene in the maize and rice chloroplast genomes, and to orf28 and orf64 in the rice chloroplast genome. Northern-blot analysis indicates that the ct-like trnI (CAT) gene is likely to be a pseudo gene in the maize cmsT mt genome because it is not transcribed. A nucleotide-sequence homology search of this 7.5-kbp region reveals several short segments homologous to portions of chloroplast (ct), mitochondrial (mt) and other known genes. These segments range from 17 bp to 187 bp in length with homology from 71 to 100%. These observations also suggest that the transfer of DNA fragments from the ct genome to the mt genome may have occurred at different times during the evolution of the maize mt genome, and that multiple recombination events and rearrangements in both mt and ct genomes have occurred after the transfer of DNA fragments from the ct to the mt genome. A few segments were identified by their homology to be portions of genes from sources other than the ct genome. While it is known that large fragments of ctDNA have been transferred to maize mt genomes, this is the first report of the presence of numerous short ctDNA, or other foreign DNA segments, in the maize cmsT mt genome. These segments may provide valuable information regarding the evolution of plant mt genomes.