Overlapping divergent genes in the maize chloroplast chromosome and in vitro transcription of the gene for tRNA

Abstract

In the presence of the S polypeptide, maize chloroplast DNA-dependent RNA polymerase preferentially transcribes sequences within the 2200-nucleotide-pair-long maize chloroplast chromosome fragment Eco [unk] from a supercoiled chimeric plasmid cloned in Escherichia coli [Jolly, S. O. & Bogorad, L. (1980) Proc. Natl. Acad. Sci. USA 77, 822-826]. Eco [unk] contains one gene for tRNA(His) and one for a 1.6-kilobase RNA that includes an open reading frame. These two genes overlap by at least a few nucleotides and are transcribed divergently from complementary DNA strands. This indicates possible transcriptional regulation of chloroplast DNA at the nucleotide level. The 5' end of tRNA(His) (G-U-G) isolated from maize chloroplasts is indistinguishable from that of the transcript produced from Eco [unk] in vitro by maize chloroplast DNA-dependent RNA polymerase. This purified system initiates RNA synthesis faithfully and exhibits preference for some chloroplast genes. Maize chloroplast DNA for tRNA(His) lacks the sequence C-C-A at its 3' terminus; it is presumably added post-transcriptionally. Maize tRNA(His) has both prokaryotic and eukaryotic features.