Fructose 1,6-bisphosphatase (FBPase) and phosphoribulokinase (PRK) are two key enzymes of the reductive pentose phosphate pathway or Calvin cycle of photosynthetic carbon dioxide assimilation. Early studies had indicated that the properties of enzymes isolated from photosynthetic bacteria were clearly distinct from those of enzymes obtained from the chloroplasts of higher plants [for a review, see Tabita (1988)]. The eucaryotic enzymes, which are light activated by the thioredoxin/ferredoxin system (Buchanan, 1980), were each shown to contain a putative regulatory amino acid sequence (Marcus et al., 1988; Porter et al., 1988). The enzymes from photosynthetic bacteria are not controlled by the thioredoxin/ferredoxin system but exhibit complex kinetic properties and, in the case of PRK, there is an absolute requirement of NADH for activity. In the photosynthetic bacterium Rhodobacter sphaeroides, the structural genes of the Calvin cycle, including the genes that encode FBPase (fbp) and PRK (prk), are found in two distinct clusters, and the fbp and prk genes are closely associated in each cluster. In the present investigation, we have determined the nucleotide sequence of the fbpB and prkB genes of the form II cluster and have compared the deduced amino acid sequences to previously determined sequences of light-activated enzymes from higher plants and from other eucaryotic and procaryotic sources. In the case of FBPase, there are several regions that are conserved in the R. sphaeroides enzymes, including a protease-sensitive area located in a region equivalent to residues 51-71 of mammalian FBPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Hematopoietic progenitor cells (HPC) from mice nullizygous at the Fanconi anemia (FA) group C locus (FAC −/−) are hypersensitive to the mitotic inhibitory effects of interferon (IFN-γ). We tested the hypothesis that HPC from the bone marrow of Fanconi group C children are similarly hypersensitive and that the fas pathway is involved in affecting programmed cell death in response to low doses of IFN-γ. In normal human and murine HPC, IFN-γ primed the fas pathway and induced both fas and interferon response factor-1 (IRF-1) gene expression. These IFN-γ-induced apoptotic responses in HPC from the marrow of a child with FA of the C group (FA-C) and in FAC −/− mice occurred at significantly lower IFN doses (by an order of magnitude) than did the apoptotic responses of normal HPC. Treatment of FA-C CD34+ cells with low doses of recombinant IFN-γ, inhibited growth of colony forming unit granulocyte-macrophage and burst-forming unit erythroid, while treatment with blocking antibodies to fas augmented clonal growth and abrogated the clonal inhibitory effect of IFN-γ. Transfer of the normal FAC gene into FA-C B-cell lines prevented mitomycin C–induced apoptosis, but did not suppress fas expression or inhibit the primed fas pathway. However, the kinetics of Stat1-phosphate decay in IFN-γ–treated cells was prolonged in mutant cells and was normalized by transduction of the normal FAC gene. Therefore, the normal FAC protein serves, in part, to modulate IFN-γ signals. HPC bearing inactivating mutations of FAC fail to normally modulate IFN-γ signals and, as a result, undergo apoptosis executed through the fas pathway.

Two enzymes in the methionine salvage pathway, 5-methylthioribose kinase (MTR kinase) and 5'-methylthioadenosine/ S-adenosylhomocysteine nucleosidase (MTA/SAH nucleosidase) were purified from Klebsiella pneumoniae. Chromatography using a novel 5'-(p-aminophenyl)thioadenosine/5-(p-aminophenyl)thioribose affinity matrix allowed the binding and selective elution of each of the enzymes in pure form. The molecular mass, substrate kinetics and N-terminal amino acid sequences were characterized for each of the enzymes. Purified MTR kinase exhibits an apparent molecular mass of 46-50 kDa by SDS/PAGE and S200HR chromatography, and has a Km for MTR of 12.2 microM. Homogeneous MTA/SAH nucleosidase displays a molecular mass of 26.5 kDa by SDS/PAGE, and a Km for MTA of 8.7 microM. Comparisons of the N-terminal sequences obtained for each of the enzymes with protein-sequence databases failed to reveal any significant sequence similarities to known proteins. However, the amino acid sequence obtained for the nucleosidase did share a high degree of sequence similarity with the putative translation product of an open reading frame in Escherichia coli, thus providing a tentative identification of this gene as encoding an MTA/SAH nucleosidase.

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The Fanconi anemia (FA) complementation group C (FAC) protein gene encodes a cytoplasmic protein with a predicted Mrof 63,000. The protein's function is unknown, but it has been hypothesized that it either mediates resistance to DNA cross-linking agents or facilitates repair after exposure to such factors. The protein also plays a permissive role in the growth of colony-forming unit–granulocyte/macrophage (CFU-GM), burst-forming unit–erythroid (BFU-E), and CFU-erythroid (CFU-E). Attributing a specific function to this protein requires an understanding of its intracellular location. Recognizing that prior study has established the functional importance of its cytoplasmic location, we tested the hypothesis that FAC protein can also be found in the nucleus. Purified recombinant Escherichia coli–derived FAC antigens were used to create antisera able to specifically identify an Mr = 58,000 protein in lysates from human Epstein-Barr virus (EBV)-transformed cell lines by immunoblot analysis. Subcellular fractionation of the cell lysates followed by immunoblot analysis revealed that the majority of the FAC protein was cytoplasmic, as reported previously; however, approximately 10% of FAC protein was reproducibly detected in nuclear fractions. These results were reproducible by two different fractionation methods, and included markers to control for contamination of nuclear fractions by cytoplasmic proteins. Moreover, confocal image analysis of human 293 cells engineered to express FAC clearly demonstrated that FAC protein is located in both cytoplasmic and nuclear compartments, consistent with data obtained from fractionation of the FA cell lines. Finally, complementation of the FAC defect using retroviral-mediated gene transfer resulted in a substantial increase in nuclear FAC protein. Therefore, while cytoplasmic localization of this protein appears to be functionally important, it may also exert some essential nuclear function.

5-Methylthioribose (MTR) kinase catalyses a key step in the recycling of methionine from 5'-methylthioadenosine, a co-product of polyamine biosynthesis, in Klebsiella pneumoniae. In defined medium lacking methionine, K. pneumoniae exhibits abundant MTR kinase activity. When the bacterium is transferred to a medium containing 10 mM-methionine, the specific activity of MTR kinase decreases in a fashion consistent with repression of new enzyme synthesis and dilution of existing enzyme by cell division. The specific activity of methionine synthase decreases to a similar degree under the same conditions. In Escherichia coli and Salmonella typhimurium, the gene for methionine synthase is co-ordinately controlled as part of the methionine regulon. Taken together, our results indicate that a methionine regulon may function in K. pneumoniae and that expression of MTR kinase may be under its control.

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