Transcript analysis of the tobacco plastid operon rps2/atpI/H/F/A reveals the existence of a non-consensus type II (NCII) promoter upstream of the atpI coding sequence

Abstract

The plastid ATP synthase complex is composed of nine subunits, of which six are encoded in the plastome. The plastid-encoded genes are arranged in two transcriptional units: atpB/E and atpI/H/F/A. We have recently reported that besides containing four −10 and −35 consensus-type (CT) promoters, the atpB/E operon also contains a non-consensus type (NCII) promoter that alone is responsible for its expression in non-photosynthetic plastids. As the functionality of ATP synthase requires expression of all nine subunits, NCII promoter-driven transcription of the atpI/H/F/A operon is to be expected in non-photosynthetic plastids. Therefore, a detailed transcriptional analysis of this operon was carried out using RNA samples from tobacco leaf, cultured cells (BY-2) and seedlings grown on streptomycin and spectinomycin; which contain chloroplasts, translationally active non-photosynthetic plastids and translationally inactive plastids, respectively. We identified a total of three transcription initiation sites (TIS) and four transcript processing sites in the non-coding regions of this operon. Our results also demonstrate that rps2 is co-transcribed with the atpI/H/F/A genes. One of the TIS (−208 atpI) is characterized by an NCII type promoter, while other two primary transcripts (−131 atpI and −384 atpH) initiate from CT promoters. In non-photosynthetic plastids the atpI/H/F/A-specific transcript pool seems to be solely contributed by initiation at the −208 atpI (NCII type) promoter, because transcripts from CT promoters do not accumulate in these plastid types.