The mycobacterial guaB1 gene encodes a guanosine 5'-monophosphate reductase with a cystathionine-ß-synthase domain.

Mycobacteria express enzymes from both the de novo and purine-salvage pathways. However, the regulation of these processes and the roles of individual metabolic enzymes have not been sufficiently detailed. Both Mycobacterium tuberculosis (Mtb) and Mycobacterium smegmatis (Msm) possess three guaB genes, but information is only available on guaB2, which encodes an essential inosine 5'-monophosphate dehydrogenase (IMPDH) involved in de novo purine biosynthesis. This study shows that guaB1, annotated in databases as a putative IMPDH, encodes a guanosine 5'-monophosphate reductase (GMPR), which recycles guanosine monophosphate to inosine monophosphate within the purine-salvage pathway and contains a cystathionine-ß-synthase domain (CBS), which is essential for enzyme activity. GMPR activity is allosterically regulated by the ATP/GTP ratio in a pH-dependent manner. Bioinformatic analysis has indicated the presence of GMPRs containing CBS domains across the entire Actinobacteria phylum.

Hypoxanthine-Guanine Phosphoribosyltransferase Is Dispensable for Mycobacterium smegmatis Viability.

Purine metabolism plays a ubiquitous role in the physiology of Mycobacterium tuberculosis and other mycobacteria. The purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is essential for M. tuberculosis growth in vitro; however, its precise role in M. tuberculosis physiology is unclear. Membrane-permeable prodrugs of specifically designed HGPRT inhibitors arrest the growth of M. tuberculosis and represent potential new antituberculosis compounds. Here, we investigated the purine salvage pathway in the model organism Mycobacterium smegmatis Using genomic deletion analysis, we confirmed that HGPRT is the only guanine and hypoxanthine salvage enzyme in M. smegmatis but is not required for in vitro growth of this mycobacterium or survival under long-term stationary-phase conditions. We also found that prodrugs of M. tuberculosis HGPRT inhibitors displayed an unexpected antimicrobial activity against M. smegmatis that is independent of HGPRT. Our data point to a different mode of mechanism of action for these inhibitors than was originally proposed.IMPORTANCE Purine bases, released by the hydrolytic and phosphorolytic degradation of nucleic acids and nucleotides, can be salvaged and recycled. The hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which catalyzes the formation of guanosine-5'-monophosphate from guanine and inosine-5'-monophosphate from hypoxanthine, represents a potential target for specific inhibitor development. Deletion of the HGPRT gene (Δhgprt) in the model organism Mycobacterium smegmatis confirmed that this enzyme is not essential for M. smegmatis growth. Prodrugs of acyclic nucleoside phosphonates (ANPs), originally designed against HGPRT from Mycobacterium tuberculosis, displayed anti-M. smegmatis activities comparable to those obtained for M. tuberculosis but also inhibited the ΔhgprtM. smegmatis strain. These results confirmed that ANPs act in M. smegmatis by a mechanism independent of HGPRT.

Catabolism of 8-oxo-purines is mainly routed via the guanine to xanthine interconversion pathway in Mycobacterium smegmatis.

Metabolism of purine bases remains poorly understood in the pathogenic bacterium Mycobacterium tuberculosis and closely related, nonpathogenic Mycobacterium smegmatis (Msm). To gain insight into the purine metabolism in mycobacteria, we tested uptake of purine bases with a ΔpurF Msm mutant with an inactive purine de novo biosynthesis pathway and confirmed that hypoxanthine and guanine, but not xanthine, can serve as nucleotide precursors for recycling in the salvage pathway. Further, we focused on purine catabolism in wild-type (wt) Msm. We found that only xanthine and guanine could serve as a sole nitrogen source for wt Msm. These data confirm that Msm catabolism of purines is directed mainly via oxidative guanine to xanthine interconversion and not through metabolic conversion of hypoxanthine to xanthine. Our data represent the first experimental evidence confirming the use of 8-oxo-purines as a nitrogen source by Msm.

Decolorization and detoxification of textile wastewaters by recombinant Myceliophthora thermophila and Trametes trogii laccases.

Laccases are multi-copper oxidoreductases with broad biotechnological applications. Here, we report detailed biochemical characterization of purified recombinant laccases originating from Myceliophthora thermophila (MtL) and Trametes trogii (TtL). We identified optimal conditions for decolorization of commercial dyes and textile wastewater samples. We also tested the toxicity of decolorized wastewater samples using human peripheral blood mononuclear cells. MtL and TtL were expressed in Saccharomyces cerevisiae, and secreted enzymes were purified by consecutive hydrophobic and gel chromatography. The molecular masses of TtL (~ 65 kDa) and MtL (> 100 kDa) suggested glycosylation of the recombinant enzymes. Deglycosylation of MtL and TtL led to 25% and 10% decreases in activity, respectively. In a thermal stability assay, TtL retained 61% and MtL 86% of the initial activity at 40 °C. While TtL retained roughly 50% activity at 60 °C, MtL lost stability at temperatures higher than 40 °C. MtL and TtL preferred syringaldazine as a substrate, and the catalytic efficiencies for ABTS oxidation were 7.5 times lower than for syringaldazine oxidation. In the presence of the mediator HBT, purified TtL almost completely decolorized dyes within 30 min and substantially decolorized wastewater samples from a textile factory (up to 74%) within 20 h. However, products of TtL-catalyzed decolorization were more toxic than MtL-decolorized products, which were almost completely detoxified.

Efficient secretion of three fungal laccases from Saccharomyces cerevisiae and their potential for decolorization of textile industry effluent-A comparative study.

Laccases are enzymes with a broad range of biotechnological applications and have, for example, the ability to oxidize many xenobiotics including synthetic dyes. In order to obtain an efficient laccase for the decolorization of dyes which spoil wastewater from the textile industry, genes encoding three various laccase enzymes were expressed in Saccharomyces cerevisiae. The expression of laccases from ascomycete Myceliophthora thermophila (MtL), and two basidiomycetes Trametes versicolor (TvL) and Trametes trogii (TtL) was optimized via selection of plasmids, promoters, media composition, and cultivation conditions. For the first time, the activity of the three secreted laccases was directly compared with the use of various substrates, including different dyes and a wastewater sample. A strong constitutive ADH1 promoter, minimal growth medium, optimized combination of copper and organic nitrogen source, and low cultivation temperature were shown to significantly increase the yields and relative activities of secreted laccases. Heterologous expression of three fungal laccases was successfully achieved in S. cerevisiae being the highest for MtL and the lowest for TvL. MtL, and particularly TtL, showed the decolorization capacity. This is the first report which compared decolorization of synthetic dyes and wastewater by several recombinant laccases and suggested MtL and TtL to be applicable in the ecofriendly enzymatic treatment of colored industry effluent. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 34:69-80, 2018.

Immobilization in polyvinyl alcohol hydrogel enhances yeast storage stability and reusability of recombinant laccase-producing S. cerevisiae.

OBJECTIVES: To improve the storage stability and reusability of various yeast strains and species by immobilization in polyvinyl alcohol (PVA) hydrogel particles. RESULTS: Debaryomyces hansenii, Pichia sorbitophila, Saccharomyces cerevisiae, Yarrowia lipolytica, and Zygosaccharomyces rouxii were immobilized in PVA particles using LentiKats technology and stored in sterile water at 4 °C. The immobilization improved the survival of all species; however, the highest storage stability was achieved for S. cerevisiae and Y. lipolytica which survived more than 1 year, in contrast to free cells that survived for only 3 months. Tests of the reusability of immobilized recombinant laccase-secreting S. cerevisiae revealed that the cells were suitable for repetitive use (55 cycles during 15 months) even after storage in water at 4 °C for 9 months. A suitable method for killing immobilized laccase-secreting cells without affecting the produced enzyme activity was also developed. CONCLUSIONS: The immobilization of yeasts in PVA hydrogel enables long-term, cheap storage with very good cell viability and productivity, thus becoming a promising approach for industrial applications.