Identification of a cis-acting element controlling anaerobic expression of the hydA gene from Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, the expression of the hydA gene encoding the [Fe]-hydrogenase is controlled on the transcriptional level under anaerobic conditions. We have isolated a DNA fragment of 1819 by upstream of the hydA transcription start site by Genome Walking. To understand the mechanism that underlies the anoxic regulation in C. reinhardtii, we have fused the promoter region of hydA to the arylsulphatase (ars) reporter gene lacking its own promoter. Several deletions within the region -1819 to -21 were introduced into a cell-wall deficient alga strain. Transformants expressing the Ars enzyme in sulphur sufficient medium were isolated and shown to transcribe the hydA/ars construct only under anaerobic conditions. The expression of the ars gene was high enough to allow quantitative measurements. The results indicate that the region between positions -128 and -21 with respect to the transcription start site is required for anaerobic specific gene expression and possibly contains at least one enhancer element.