The expression of parA, an auxin-regulated gene expressed during the culture of tobacco (Nicotiana tabacum L.) mesophyll protoplasts, is induced by cadmium. To identify the cadmium-responsive element, we examined the parA promoter using the GUS reporter gene. Cadmium responsiveness was retained in a 5' deletion of the parA promoter to -78 bp, but it was nullified by further deletion to -49bp, which implies that the region -49 to -78 bp contained a cadmium-responsive element. This region contains a sequence similar to as-1, an enhancer sequence from the cauliflower mosaic virus 35S RNA promoter that binds the nuclear factor ASF-1. We named the sequence in the parA promoter pas. Gel-shift assays revealed that pas and as-1 compete for the same DNA-binding nuclear protein(s). Since pentamers of either pas and as-1 were able to confer cadmium responsiveness on a minimal promoter but mutant as-1 was not, we propose that pas and as-1 are involved in cadmium-responsive gene expression. Neither pas nor as-1 conferred responsiveness to copper. The specificity of this response, involving the function of as-1-related elements including pas, is discussed
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