RESUMEN
A Bacillus sp. RE was resistant to chromium and reduced Cr(VI) without accumulating chromium inside the cell. When Cr(VI) was 10 and 40 microg ml(-1), >95% of the total Cr(VI) was reduced in 24 and 72 h of growth, respectively, whereas at 80 microg Cr(VI) ml(-1 )only 50% of Cr(VI) was reduced. However growth was not affected; the cell mass was 0.7-0.8 mg ml(-1) in all cases. The cell-free extract showed Cr(VI) reducing enzyme activity which was enhanced (>5 fold) by NADH and NADPH. Like whole cells the enzyme also reduced Cr(VI) with decreasing efficiency on increasing Cr(VI) concentration. The enzyme activity was optimal at pH 6.0 and 30 degrees C. The enzyme was stable up to 30 degrees C and from pH 5.5 to 8, but from pH 4 to 5 the enzyme was severely destabilized. Its Km and Vmax were 14 microM: and 3.8 nmol min(-1) mg(-1) respectively. The enzyme activity was enhanced by Cu2+ and Ni2+ and inhibited by Hg2+.