Transcription of tRNA genes from a large-scale plastid DNA deletion clearly reveals the action of nuclear-encoded RNA polymerase in the plastid

We have recently characterized large-scale deleted plastid DNAs in calli derived From rice (Oryza sativa) anther culture. The size and location of these deletions differed between the calli. In this study the conformation of the largest plastid DNA deletion of callus A10 was studied using neutral-alkaline, two-dimensional, agarose-gel electrophoresis. This clearly revealed that its DNA had the same conformation as that of-the previously described callus A5: linear molecules with a hairpin structure at both termini and existing in both monomer and multimer forms linked to form head-to-head and tail-to-tail concatemers. The region retained by both A10 and A5 was only about 3 kbp in size and encoded trnfM, trnG, phi trnI, trnI; trnI; and trnE. Transcripts from these deleted ptDNAs were identified by RNA gel blotting using oligonucleotide DNA probes. These results clearly indicated the involvement of nuclear-encoded RNA polymerase in the transcription of-the tRNA genes because these plastid genomes lack nearly all the genes encoding their own translational apparatus (i.e. rRNA and tRNA). The A10 plasmid also lacks all RNA polymerase genes (rpoA, Band C). The function of the large-scale plastid DNA deletion is discussed.