Transcription of a light-inducible gene in the prokaryote Arthrobacter sp. is induced in the dark when cells are incubated with chelating agents or in medium at pH 5. However, repletion of metal ions such as Ca2+, Mn2+ or Zn2+ or an increase in pH is required for accumulation of the gene product, an Mr 21,000 polypeptide. But such changes in condition restore repression of the gene, and the decay in the rate of synthesis of the polypeptide follows the same time-course as when photodynamically induced cells are transferred to the dark. These results are consistent with regulation of expression of this gene at transcriptional and posttranscriptional steps by mechanisms that involve metal-protein complexes.