Expression of the psbDII gene in Synechococcus sp. strain PCC 7942 requires sequences downstream of the transcription start site.

The psbDI and psbDII genes in Synechococcus sp. strain PCC 7942 encode the D2 polypeptide, an essential component of the photosystem II reaction center. Previous studies have demonstrated that transcripts from psbDII, but not psbDI, increase in response to high light intensity. Soluble proteins from Synechococcus cells shifted to high light were found to have affinity for DNA sequences upstream from the psbDII coding region. DNA mobility-shift and copper-phenanthroline footprinting assays of a 258-bp fragment revealed three distinct DNA-protein complexes that mapped to the untranslated leader region between +11 and +84. Deletion of the upstream flanking region to -42 had no effect on the expression of a psbDII-lacZ reporter gene or its induction by light, whereas a promoterless construct supported only minimal background levels of beta-galactosidase. A 4-bp deletion within the first protected region of the footprint decreased the beta-galactosidase activity to approximately 2% of that of the undeleted control, but gene expression remained responsive to light. Deletion of the three protected regions completely abolished both gene expression and light induction. These results suggest that the psbDII gene requires elements within the untranslated leader region for efficient gene expression, one of which may be involved in regulation by light.