To determine whether chromate reduction requires the physical presence of E. cloacae cells, cells were grown anaerobically in KSC medium with 0.5 mM K2CrO4 and then removed from the culture by filtration through membrane filters (0.45-,um pore size). The composition of KSC medium has been previously described (13). Sodium acetate was added as a carbon source and electron donor for chromate reduction, and the pH of the medium was 7.4. The cell-free filtrate did not reduce CrO42-even under anaerobic conditions (Fig. 1A). Cells were also removed by centrifugation at 13,000 x g for 10 min; no chromate reduction was observed in the supernatant. Adding fresh KSC medium to the supernatant did not cause chromate reduction. On the contrary, when washed cells were suspended in fresh KSC medium (about 2 x 107 cells per ml), chromate reduction was readily observed. They were able to reduce completely concentrations as high as 0.5 mM CrO42-within 5 h; such a high rate of chromate reduction has never been reported.This reductase activity was eliminated by aeration (Fig. iB). No chromate reduction was observed at 4 or 60°C.

Two cDNAs encoding casein kinase-1 have been isolated from a yeast cDNA library and termed CKI1 and CKI2. Each clone encodes a protein of approximately 62,000 Da containing a highly conserved protein kinase domain surrounded by variable amino- and carboxy-terminal domains. The proteins also contain two conserved carboxy-terminal cysteine residues that comprise a consensus sequence for prenylation. Consistent with this posttranslational modification, cell fractionation experiments demonstrate that intact CKI1 is found exclusively in yeast cell membranes. Gene disruption experiments reveal that, although neither of the two CKI genes is essential by itself, at least one CKI gene is required for yeast cell viability. Spores deficient in both CKI1 and CKI2 fail to grow and, therefore, either fail to germinate or arrest as small cells before bud emergence. These results suggest that casein kinase-1, which is distributed widely in nature, plays a pivotal role in eukaryotic cell regulation.