Differential Responses of Groel and Metallothionein Genes to Divalent Metal Cations and the Oxyanions of Arsenic in the Cyanobacterium Synechococcus Sp. Strain Pcc7942

A better understanding of the resistance mechanisms used by the cyanobacterium Synechococcus sp. PCC 7942 could guide attempts to develop this organism for use in bioremediation. This bacterium exhibits extraordinary resistance to arsenate. Possible mechanisms include exclusion from the cell, intraand extra-cellular binding, and potential interactions with proteins such as metallothioneins, which bind and detoxify divalent metal cations such as Cd2+, Cu2+, and Zn2+. Synechococcus sp. PCC 7942 expresses messenger RNA for metallothioneins in response to these divalent metal ions at various concentrations. Interestingly, a similar pattern of expression is induced in response to the oxyanions of arsenic, arsenate, and arsenite. The response to arsenate, however, occurs only when cells are cultured in phosphate-limited media. This suggests that under phosphate-sufficient conditions, arsenate is specifically excluded from the cell. Our hypothesis is that arsenate, upon entering phosphate-depleted cells, interacts with the sulfhydryl groups of the cysteine residues of the regulatory protein responsible for expression of the metallothionein gene. A thiol-alkylating reagent induced metallothionein synthesis, presumably by modifying the sulfhydryl residues of the regulatory protein, consistent with our proposed mechanism for arsenate induction of metallothionein expression. We further describe the effects of divalent metal cations and the oxyanions of arsenic on induction of the general stress protein GroEL. We have found that transcription of this gene begins within 15 minutes of exposure to the divalent metal cations Cd2+, Cu2+, and Zn2+ as well as to arsenate and arsenite. Thus, conditions that induce expression of the general stress protein GroEL and the metal-binding protein metallothionein are somewhat similar.