New pleiotropic mutants were isolated that express either the phoA, psiE or psiO promoter constitutively and simultaneously alter bacterial alkaline phosphatase regulation, carbon utilization or ultraviolet light sensitivity. To do this, Lac+ mutants were isolated from strains with the appropriate lacZ transcriptional fusions. Over 300 independent mutants were characterized, and all that constitutively express phoA map in phoR, phoU, the phosphate-specific transport system or a new locus called phoF. However, only phoU mutants express both phoA and psiE constitutively. Carbohydrate-utilizing mutants that show constitutive expression of psiE and psiO map in cya, crp and, possibly, crr. Also, numerous ultraviolet-light-sensitive mutants were discovered that show increased psiO expression and map in lon. Some other mutations that lead to constitutive psiO expression (which is normally induced either by phosphate, nitrogen or carbon starvation or anoxia) show decreased expression of phoA. Also, several mutants were found that show an unusual metastable character affecting psiO or phoA transcription. In these, colonies spontaneously switch between an induced and repressed "state" with respect to lac or bacterial alkaline phosphatase expression. In some, the clonal variation of the lactose phenotype or bacterial alkaline phosphatase synthesis is recA-independent and phenotypically resembles phase variation in Salmonella typhimurium. The latter class are called "phase mutants". The mutants are discussed in terms of protein-nucleic acid interactions and/or possible changes in the DNA, i.e. modifications or rearrangements, within the phosphate gene system, that are physiologically regulated.