Small single-copy region of plastid DNA in the non-photosynthetic angiosperm Epifagus virginiana contains only two genes : Differences among dicots, monocots and bryophytes in gene organization at a non-bioenergetic locus

Abstract

We have determined the nucleotide sequence of a 7 kb (1 kb = 10 3 base-pairs) region that includes the entire small single-copy region (SSC) of the plastid genome of Epifagus virginiana, a non-photosynthetic, parasitic flowering plant. The SSC (4.8 kb) is considerably smaller than those of photosynthetic plants due to the complete deletion of all photosynthetic, chlororespiratory and ribosomal protein genes. This leaves only two genes: a protein gene of 1738 codons whose product is unlikely to be involved in bioenergetic processes and a leucine tRNA gene ( trnL UAG). Both genes span junctions between the inverted repeat and the SSC, with the consequence that the terminal 20 base-pairs of the repeat is transcribed in both directions and functions both as the 3′ end of the tRNA gene and as an internal segment of orf1738. We find that the region of tobacco plastid DNA homologous to Epifagus orf1738 contains a single open reading frame (ORF) of 1901 codons rather than the three ORFs of 1244, 273 and 228 codons originally reported. However, we confirm that the equivalent region of the bryophyte Marchantia contains two genes (1068 and 464 codons) corresponding to the N and C-terminal portions of the dicot protein. In contrast, rice plastid DNA contains a severely truncated pseudogene at this locus.