A conserved His-Asp signal response regulator-like gene in Heterosigma akashiwo chloroplasts

Regulation of gene expression in plastids may involve molecular components conserved from cyanobacteria-like ancestors. Among prokaryotes, genes are commonly regulated at the transcriptional level by 'two-component' or 'His-Asp' signal transducers, consisting of a 'sensor kinase', which autophosphorylates at a conserved histidine residue, and a cognate response regulator, which is phosphorylated by the sensor kinase at a conserved aspartate residue. A putative His-Asp response regulator gene (trg1: transcriptional regulatory gene 1) has been identified in the estuarine raphidophytic alga Heterosigma akashiwo. The chloroplast-encoded trg1 is 693 bp in length, contains no introns, and yields a conceptual translation product of 231 amino acids, with a predicted mass of 27 kDa. Homology searches suggest that Heterosigma trgl has an omnpR-like identity within the DNA-binding His-Asp family of response regulators. trg1 contains both the phosphorylation and DNA-binding domains which are present in prokaryote response regulators. Quantitative competitive RT-PCR showed that Heterosigma trg1 is expressed at low levels (5 microg per g total RNA). In contrast, psbA (a photosystem II component) transcript is abundant (60 mg per g total RNA). Cell cycle analysis showed that psbA abundance oscillates in response to light but trg1 mRNA levels are invariant. We hypothesize that a His-Asp phosphorelay mechanism may affect chloroplast genome transcription in a manner similar to bacterial signal transduction pathways in which 'sensor kinase' and cognate 'response regulator' proteins interact.