Molecular analysis of the linear 2.3 kb plasmid of maize mitochondria: apparent capture of tRNA genes

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Abstract
The nucleotide sequence and transcription pattern of the linear 2.3 kb plasmid of maize mitochondria was analyzed in order to elucidate its possible function in the organelle. The plasmid has 170 kb inverted repeats at its termini composed, in turn, of shorter repetitive sequences. An open reading frame within the plasmid is transcribed and can potentially specify by a 33 kD product. In addition the plsmid contains two tRNA genes homologous to chloroplast sequences; the tRNApro(CAA) and the tRNAtrp(UGG). Both of the tRNA genes of the plasmid are transcribed, but apparently only the tRNAtrp is processed to other correct size. These tRNA sequences are found in the main mitochodnrial genome of all higher plants tested, and in most maize relatives. An exception is the close maize relative Northern teosinte in which the tRNAtrp gene is also caried on a plasmid. Thees results suggest that the 2.3 kb plasmid has acquired the tRNA sequences from the main mitochondrial DNA. It is possible that the plasmid-encoded tRNAtrp gene is essential for organelle function thereby ensuring the maintenance of the plasmid in the mitochondrion.