Organization and sequence of five tRNA genes and of an unidentified reading frame in the wheat chloroplast genome: evidence for gene rearrangements during the evolution of chloroplast genomes.

The genes for the initiator tRNA(Met)CAU, tRNA(Gly)UCC, tRNA(Thr)GGU, tRNA(Glu)UUC and tRNA(Tyr)GUA and an open reading frame of 62 codons have been identified by sequencing a 2,358 bp BamHI and a 1,378 bp BamHI-Sst2 DNA fragments from wheat chloroplasts. A comparison of the organization of these five tRNA genes and of the open reading frame on the wheat, tobacco and spinach chloroplast genomes suggests that at least three genomic inversions must have occurred during the evolution of the wheat chloroplast genome from a spinach-like ancestor genome. Furthermore, it seems that in wheat the 91 bp intergenic region between the genes for the initiator tRNA(Met) and the gene for tRNA(Gly)UCC is one end-point of the 20 kbp genomic inversion proposed by Palmer and Thompson in the case of maize (Palmer and Thompson 1982). A 119 bp duplication is located at this junction: the first copy comprises the 91 bp of the intergenic region and the first 28 bp of the tRNA(Met) gene, the second copy is found downstream of the tRNA(Met) gene.