Transcription and the architecture of promoters in chloroplasts

Abstract

Post-transcriptional processes play an important role in the regulation of chloroplast gene expression1. Recent experimental data indicate that plastid transcription is also extensively regulated. Indeed, the process responsible for this in chloroplasts might be even more complex than in the cyanobacterial ancestors of these organelles. Substantial evidence has accumulated that gene expression in the plastids of higher plants relies on two different transcriptional machineries: one dependent on the plastid-encoded subunits (rpoA, rpoB, rpoC1 and rpoC2) of a eubacterial-like RNA polymerase forming the core of the plastid-encoded RNA polymerase (PEP); and a second, nuclear-encoded RNA polymerase (NEP) (Refs 2,3). PEP exists, at least in mustard (Sinapis alba), in a rifampicin-sensitive and a rifampicin-resistant form4. Moreover, the detection of nuclear genes encoding at least five different chloroplast sigma factors (which are thought to facilitate the binding of PEP to distinct classes of promoters) indicates a highly complex transcription apparatus of the eubacterial type. Recently, the nature and the functional role of the nuclear-encoded RNA polymerase, NEP, has also become clearer. An early indication that NEP might be a single-subunit phage-type RNA polymerase came from biochemical characterization of partially purified transcriptional activity from spinach chloroplasts5. These recent studies have convincingly demonstrated the existence of NEP, and also characterized the promoters it uses.