Genetic selection scheme for isolation of signal transduction pathway mutants.

Genetic characterization of a signal transduction pathway requires the isolation of mutations in the pathway. Characterization of these mutated genes and their loci enumerates the components of the pathway and leads to an understanding of the role of each gene locus in the pathway under study. We have designed and developed a strategy based on resistance to the chemical flucytosine for the identification of mutations in a given pathway. In this study, the Escherichia coli codA gene, which encodes the enzyme cytosine deaminase, was fused to the light-intensity-regulated gene promoter psbDII. Cytosine deaminase converts 5'-fluorocytosine to the toxic product 5-fluorouracil. Wild-type cells containing an intact signal transduction pathway that regulates the psbDII promoter will die in the presence of this chemical. Cells that carry mutations in the pathway that inactivate the psbDII promoter will not express the codA gene and, consequently, will live on 5'-fluorocytosine, allowing the isolation and subsequent characterization of mutations in this signaling pathway. Utilizing this selection method, we have successfully isolated and characterized mutations in the psbDII pathway. This selection scheme can be used with a tissue-specific or phase-specific promoter fused to the codA gene to direct the timing of expression of codA to obtain mutants defective in temporal or cell-specific expression of a particular pathway. This scheme also allows the isolation of mutants even when a clearly identifiable phenotype is not available. The selection scheme presented here extends the molecular tools available for the genetic dissection of signal transduction pathways.