An in vitro transcription termination system to analyze chloroplast promoters: identification of multiple promoters for the spinach atpB gene.

Promoters for spinach chloroplast genes were cloned 5' to a strong factor-independent transcription terminator from E. coli. These "minigene" constructions were transcribed in vitro by a transcriptionally active extract of spinach chloroplasts. Transcription of supercoiled DNA templates resulted in synthesis of discretely-sized RNAs that were readily quantifiable. The efficiency of transcription was up to 3.5 RNAs per template. The transcription termination system described in this report was used to identify the primary transcripts for the plastid atpB gene. Four in vivo transcripts for the atpB gene have been previously identified with 5' untranslated leaders of approximately 455, 275, 180 and 100 nucleotides, respectively. In this report we show that the "-455", "-275" and "-180" regions function as chloroplast promoters in vitro. In addition, a fourth promoter was found that yields a primary transcript totally lacking an untranslated leader.