In the genetic disease _____ thymine dimer excision repair is deficient in UV irradiated cells. Penicillin kills growing E. coli cells but not cells that are not growing. Therefore, one can use penicillin enrichment and _____ to isolate mutants. Bacteria regulate biosynthetic pathways by _____ and _____. _____ is the process by which some cells are incapable of receiving an exogenote.