Isolation of the Anabaena sp. strain PCC 7120 sigA gene in a transcriptional-interference selection.

A transcriptional-interference selection was performed to identify genes of Anabaena sp. strain PCC 7120 that encode DNA-binding proteins able to bind to the rbcL promoter. Unexpectedly, the selection yielded the previously identified sigA gene, which encodes the principal sigma factor. Protein extracts from Escherichia coli containing the sigA gene bound the rbcL promoter fragment in mobility shift assays, and competition experiments indicated binding to rbcL and glnA but not xisA or nifH upstream regions.