Polarity of the cysJIH operon of Salmonella typhimurium.

Certain point and deletion mutants with lesions in the cysJ gene of Salmonella typhimurium have low levels of enzymes coded by the cysI and cysH genes. These results support the hypothesis that an operon exists comprising genes cysJ, I and H which is transcribed in the direction from cysJ to H. The nearby cysC and cysD genes do not form part of this cysJIH operon.

Characterization of a thermolabile sulfite reductase from Salmonella pullorum.

The biochemical basis for sulfite accumulation by sulfate-using revertants of Salmonella pullorum was determined. All of the sulfate-using mutants isolated from wild-type S. pullorum accumulated sulfite when grown at 37 but not at 25 C. The specific activity of reduced nicotinamide adenine dinucleotide (NADPH)-dependent sulfite reductase (H (2)S-NADP oxidoreductase, EC 1.8.1.2) and of reduced methyl viologen (MVH)-dependent sulfite reductase (H (2)S-MV oxidoreductase), in extracts prepared from cells incubated at 37 C, declined as the incubation period lengthened. However, the specific activity of both reductases from cells incubated at 25 C did not decline. Thermolability of cell-free NADPH-dependent sulfite reductase from cells of S. pullorum incubated at 37 C was greater than the lability of this enzyme either from cells of S. typhimurium incubated at 37 C or from cells of S. pullorum incubated at 25 C. Cells cultured at 37 C continued to accumulate sulfite when the incubation temperature was shifted to 25 C; cells cultured at 25 C and shifted to 37 C accumulated no sulfite, whereas these cells shifted to 41 C accumulated sulfite. It was concluded that the configuration of the sulfite reductase of S. pullorum strain 6-18 is a function of the incubation temperature at which synthesis occurs.