Induction of SOS genes of Escherichia coli by chromium compounds.

The induction of several SOS genes of Escherichia coli such as recA, umuC, and sfiA by hexavalent (K2Cr2O7, K2CrO4, and CrO3) and trivalent (CrCl3, Cr(NO3)3, and (CH3COO)3Cr) compounds of chromium was studied. Induction was measured as beta-galactosidase activity, using lacZ gene fusions under the control region of different SOS genes. The hexavalent chromium forms induced the genes responsible for massive synthesis of RecA protein, error-prone repair, and inhibition of cell division. On the other hand, the trivalent chromium compounds were unable to induce any of the SOS genes tested. Individual assay of hexavalent chromium compounds showed that K2Cr2O7 was a stronger inducing agent of those three SOS genes tested than K2CrO4, which, in turn, was stronger than CrO3. All this data led to the conclusion that hexavalent chromium compounds, but not trivalent, are proficient agents of induction of the SOS system and can produce indirect mutagenesis in Escherichia coli.