A conserved tryptophan at the ferredoxin-binding site of ferredoxin:nitrite oxidoreductase.

Treatment of spinach leaf ferredoxin-dependent nitrite reductase with N-bromosuccinimide (NBS), under conditions where slightly less than 1 mol of tryptophan is modified per mole of nitrite reductase, inhibits the catalytic activity of the enzyme by ca. 80% without any effect on substrate binding or other enzyme properties. Complex formation between nitrite reductase and ferredoxin completely protects the enzyme against this inhibition. Transient kinetic measurements show that the second-order rate constant for reduction of NBS-modified nitrite reductase by reduced ferredoxin is approximately four-fold larger than that observed for the native, unmodified enzyme. Also, reduction of NBS-modified nitrite reductase by the 5-deazariboflavin radical shows a different kinetic pattern than that observed with the native enzyme, suggesting that tryptophan modification increases access of the radical to the low-potential [4Fe-4S] cluster of the enzyme, decreases the accessibility to the siroheme group of the enzyme, or both. The tryptophan that is modified has been identified as the absolutely conserved W92. A methionine, M73, that is also modified by NBS, has been identified. The ferredoxin-binding site on spinach nitrite reductase thus appears to include W92 and perhaps M73, in addition to the previously identified R375, R556, and K436.

The ferredoxin-binding site of ferredoxin: Nitrite oxidoreductase. Differential chemical modification of the free enzyme and its complex with ferredoxin.

Spinach (Spinacea oleracea) leaf ferredoxin (Fd)-dependent nitrite reductase was treated with either the arginine-modifying reagent phenyl-glyoxal or the lysine-modifying reagent pyridoxal-5'-phosphate under conditions where only the Fd-binding affinity of the enzyme was affected and where complex formation between Fd and the enzyme prevented the inhibition by either reagent. Modification with [14C]phenylglyoxal allowed the identification of two nitrite reductase arginines, R375 and R556, that are protected by Fd against labeling. Modification of nitrite reductase with pyridoxal-5'-phosphate, followed by reduction with NaBH4, allowed the identification of a lysine, K436, that is protected by Fd against labeling. Positive charges are present at these positions in all of the Fd-dependent nitrite reductase for which sequences are available, suggesting that these amino acids are directly involved in electrostatic binding of Fd to the enzyme.

Isolation and characterization of soluble electron transfer proteins from Chromatium purpuratum.

Several soluble electron transfer proteins were isolated and characterized from the marine purple-sulfur bacterium Chromatium purpuratum. The C. purpuratum flavocytochrome c is similar in molecular mass (68 kDa) and isoelectric point (6.5) to flavocytochromes isolated from other phototrophs. Redox titrations of the flavocytochrome c hemes show two components with midpoint potential values of +15 and -120 mV, behavior similar to that observed with the flavocytochrome isolated from the thermophilic Chromatium tepidum. Moreover, N-terminal amino acid sequence analysis of both the flavin and the cytochrome subunit indicates substantial homology to the primary structure of the flavocytochrome c of Chromatium vinosum. In contrast, the C. purpuratum high-potential iron-sulfur protein (HiPIP) differs from those isolated from other photosynthetic bacteria in its relatively high midpoint potential (+390 mV) and the possibility that it exists as a dimer in solution. Two low molecular mass c-type cytochromes were also characterized. One appears to be a high-potential (+310 mV) c8-type cytochrome. Amino acid sequencing suggests that the second cytochrome may be a homologue of the low-potential cytochrome c-551, previously described in two species of Ectothiorhodospirillaceae.

The effect of N-bromosuccinimide on ferredoxin:NADP+ oxidoreductase.

Treatment of spinach leaf ferredoxin:NADP+ oxidoreductase (FNR) with N-bromosuccinimide (NBS), under conditions where approximately one tryptophan residue per enzyme was modified, resulted in a loss of between 80 and 85% of the activity of the enzyme when electron transfer from NADPH to either ferredoxin or 2,6-dichlorophenol-indophenol was measured. Amino acid analysis revealed no detectable modification by NBS of any FNR amino acids other than tryptophan. Complex formation with ferredoxin, but not with NADP+, prevented both the inhibition of activity and the modification of tryptophan caused by the treatment with NBS. Modification of one FNR tryptophan residue had no significant effect on the Km values of the enzyme for either ferredoxin or NADPH or on the binding constants for the FNR complexes with either ferredoxin or NADP+. NBS treatment had only very small effects on the absorbance and circular dichroism spectra of FNR and did not significantly affect either the oxidation-reduction midpoint potential of the FAD prosthetic group of the enzyme or inhibit the reduction of the FAD group by NADPH. These results raise the possibility that a tryptophan residue may play a role in the electron transfer between the FAD of FNR and the enzyme substrate, ferredoxin.

Prolactin augments progesterone-dependent uteroglobin gene expression by modulating promoter-binding proteins.

The sequence-specific binding of endometrial nuclear proteins to uteroglobin 200 (UG200) (-194/+9), the 203-base pair 5'-flanking region of the rabbit uteroglobin gene, and UG99 (-170/-85), a subfragment of UG200, was compared with gel shift assays. PRL + progesterone treatment of estrous rabbits produced a 6- to 7-fold increase in the primary shift compared to progesterone alone. PRL + progesterone treatment of long-term ovariectomized rabbits increased the primary shift 60% over progesterone alone, which increased the primary shift 30-fold over similarly treated estrous rabbits. The primary shift was eliminated when rabbits were treated with progesterone + estradiol benzoate (E2Bz). Changes in the steady state levels of UG mRNA paralleled changes in the intensity of the primary gel shift. Southwestern blotting revealed four proteins from progesterone-dominated endometrial nuclei that bind UG200. PRL pretreatment produced a 3- to 12-fold increase in each of the proteins. Protein binding was eliminated by E2Bz. A 100-kilodalton (kDa) protein from progesterone-dominated endometrial nuclei was UV cross-linked to UG200 and UG99. An additional protein was detected by each probe with long autoradiographic exposure. PRL pretreatment increased the 100-kDa protein, whereas covalent attachment of the 100-kDa protein to UG promoter DNA was eliminated by E2Bz. UV cross-linking in situ was used to identify the 100-kDa protein as responsible for the primary shift. Collectively, these experiments provide compelling evidence that PRL augments the progesterone-dependent transcription of the UG gene in the uterus by regulating at least one and as many as four proteins that bind to the UG promoter.