Enhancer activity of light-responsive regulatory elements in the untranslated leader regions of cyanobacterial psbA genes.

Three psbA genes encoding the D1 protein of the photosystem II reaction center are differentially expressed under different light intensities in the cyanobacterium Synechococcus sp. strain PCC 7942. Two of the three psbA genes, psbAII and psbAIII, are induced rapidly when light intensity is increased from 125 x 10(-6) mol.m-2.s-1 to 750 x 10(-6) mol.m-2.s-1. A recombinational cloning vector that carries a transcriptional lacZ reporter gene was used to characterize the controlling elements responsible for light induction. At least three distinct cis elements are present in the regulatory regions of pbsAII and psbAIII: basal promoters, comparable to Escherichia coli sigma 70 promoters in position and sequence, confer constitutive expression of the genes under both low and high light intensities; negative elements upstream of the promoters down-regulate the expression of the corresponding gene; and sequences downstream of the promoters that correspond to the untranslated leader regions of the mRNAs (+1 to +41 in psbAII and +1 to +39 in psbAIII) are responsible for increased expression under high light. When these light-responsive elements were combined with an E. coli promoter (conII) in different positions and orientations, the expression of the lacZ gene was induced 4- to 11-fold. The induction of gene expression under high light by these enhancers was position independent but orientation dependent. When the elements were combined with the conII promoter in the correct orientation, they also conferred a small but reproducible level of light-responsive expression on this E. coli promoter.