High-level chromate resistance in Arthrobacter sp. strain FB24 requires previously uncharacterized accessory genes

A chromate-sensitive mutant (strain D11) was generated by curing FB24 of its 96-kb plasmid. Elemental analysis indicated that chromate-exposed cells of strain D11 accumulated three times more chromium than strain FB24. Introduction of the CRD into strain D11 conferred chromate resistance comparable to wild-type levels, whereas deletion of specific regions of the CRD led to decreased resistance. Using real-time reverse transcriptase PCR, we show that expression of each gene within the CRD is specifically induced in response to chromate but not by lead, hydrogen peroxide or arsenate. Higher levels of chrA expression were achieved when the chrB orthologs and the WD40 repeat domain genes were present, suggesting their possible regulatory roles.Our findings indicate that chromate resistance in Arthrobacter sp. strain FB24 is due to chromate efflux through the ChrA transport protein. More importantly, new genes have been identified as having significant roles in chromate resistance. Collectively, the functional predictions of these additional genes suggest the involvement of a signal transduction system in the regulation of chromate efflux and warrants further study.Arthrobacter species are high G+C Gram positive bacteria that are prevalent in both pristine and polluted soils [1-3]. Although Arthrobacter spp. have been noted for their high levels of resistance to a variety of toxic metals [4,5], very little is known about the genetic basis or regulatory mechanisms underlying metal resistance in this genus. Arthrobacter sp. FB24 was isolated from soils contaminated with lead-chromate salts and was selected for detailed study based on its high tolerance to a wide assortment of toxic heavy metals [6-8]. Most notably, this strain can survive in the presence of 200 mM potassium chromate in dilute nutrient broth [6]. Reported resistance levels for other Arthrobacter species range from 2 to 48 mM chromate [9,10].The mechanism of chromium resistance in Arthrobacter strains remain