Induction of a stable, heritable epigenetic change by mutagenic carcinogens: a new test system.

The expression of the E. coli gal operon was set under the direct negative control of phage lambda repressor by a fusion between the gal operon an the active cI cro segment of phage lambda genoma. This system can exist in two stable but reversible epigenetic states: (A) the immune cI+ cro- gal- state (white colonies on McConkey gal plates) and (B) the nonimmune cI- cro+ gal+ state (red colonies). Transcription of this gal operon depends upon activation of the PR promoter; hence requiring the removal of lambda immunity repressor (cI) from the oR operator. Short exposure of this E. coli strain to radiation or chemical mutagens/carcinogens leads to a stable, inherited loss of the cI repressor (due to the take-over by the cro repressor, repressing the cI gene) and subsequent constitutive expression of the gal operon. This switch, from the (A) state to the (B) state, depends upon E. coli recA+ and lambda cro+ genes and is reversible upon supply of lambda cI repressor; this is consistent with the fact that cI and cro proteins are mutual repressors, i.e., only one of them can be expressed at a given time. This E. coli strain provides the easiest, most accurate and sensitive assay for all mutagenic and SOS-inducing agents.