This invention provides cyanobacteria as an alternative source of ahas and pds genes for plant transformations and for selectable markers. In particular, it provides for cyanobacteria, for example, Synechocystis, as a source of genes encoding herbicide insensitive proteins, and elements of genes for control of expression in plastids. Nucleic acid fragments, both the acetolactate synthase (ahas) large subunit and the ahas small subunit, were found to provide herbicide resistance. Also, the present invention provides novel Synechocystis mutant phytoene desaturase (PDS) gene conferring resistance to 4'-fluoro-6[(alpha,alpha,alpha,-trifluoro-m-tolyl)oxy]-picolinamide, a bleaching herbicide. The present invention provides improvements to method involving cyanobacteria for the screening of compounds, including a new high-through-put protocol that is a rapid and cost effective way to identify target site genes.