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

Summary The genes for the initiator tRNA_CAU^Met. tRNA_UCC^Gly, tRNA_GGU^Thr, tRNA_UUC^Glu and tRNA_GUA^Tyr and an open reading frame of 62 codons have been identified by sequencing a 2,358 by BamHl and a 1,378 by 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 by intergenic region between the genes for the initiator tRNA^Met and the gene for tRNA_UCC^Gly 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 by of the intergenic region and the first 28 by of the tRNA^Met gene, the second copy is found downstream of the tRNA^Met gene.