Conserved relationship between psbH and petBD genes: presence of a shared upstream element in Prochlorothrix hollandica.

Prochlorophytes are an unusual group of prokaryotic oxygenic photoautotrophs that morphologically appear to bridge the gap between cyanobacteria and the chloroplasts of eukaryotic plants. Molecular data place this group evolutionarily within the cyanobacteria, but they have a photosynthetic apparatus that is very similar to that found in chloroplasts. We have sequenced from the prochlorophyte Prochlorothrix hollandica a set of genes (psbB, psbH, petB and petD) that has a conserved organization in chloroplast genomes that is different from the organization in cyanobacterial genomes. The four genes are linked as an operon in chloroplasts, but only petB and petD are closely linked and cotranscribed in cyanobacteria. Although the prochlorophyte gene arrangement resembles that of cyanobacteria, one feature suggests the coordinated regulation of the unlinked genes. A 93 bp region of absolute conservation occurs upstream of the psbH gene and the petBD operon, near the site of transcription initiation in each gene set. This conserved element may indicate an alternative to cotranscription for achieving co-regulation of the psbH and petBD genes in the prochlorophyte.