Many bacteria produce extracellular molecules which function in cell-to-cell communication. One of these molecules, autoinducer 2 (AI-2), was first described as an extracellular signal produced by Vibrio harveyi to control luciferase expression. Subsequently, a number of bacteria have been shown to possess AI-2 activity in their culture supernatants, and bear the luxS gene product, which is required for AI-2 synthesis. In Porphyromonas gingivalis, luxS and pfs, encoding a 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase (MTA/SAH'ase), form an operon, suggesting that Sadenosylhomocysteine (SAH) or 5'-methylthioadenosine (MTA) serves as a substrate for AI-2 production. Cell-free extracts of Escherichia coli MG1655, but not DH5α (which carries a luxS frame-shift mutation) were capable of generating AI-2 activity upon addition of SAH, but not MTA. S-Ribosylhomocysteine (RH) derived from SAH also served as a substrate in E. coli MG1655 extracts. RH-supplemented cell-free extracts of Pseudomonas aeruginosa, a bacterium that lacks luxS, only generated AI-2 activity following the introduction of a plasmid containing the Por. gingivalis pfs-luxS operon. In addition, defined in vitro systems consisting of the purified LuxS proteins from Por. gingivalis, E. coli, Neisseria meningitidis or Staphylococcus aureus converted RH to homocysteine and a compound that exhibits AI-2 activity. 4-Hydroxy-5-methyl-3(2H)-furanone was identified by mass spectrometry analysis as a major product formed in this in vitro reaction. In E. coli MG1655, expression of T3SH [the bacteriophage T3 S-adenosylmethionine (SAM) hydrolase] significantly reduced AI-2 activity in culture supernatants, suggesting that AI-2 production is limited by the amount of SAH produced in SAM-dependent transmethylase reactions. The authors suggest that the LuxS protein has an important metabolic function in the recycling of SAH. They also show that Ps. aeruginosa is capable of removing AI-2 activity, implying that this molecule may act as a nutrient. In many bacteria AI-2 may in fact represent not a signal molecule but a metabolite which is released early and metabolized in the later stages of growth.

SummaryThe importance of methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase in bacteria has started to be appreciated only in the past decade. A comprehensive analysis of its various roles here demonstrates that it is an integral component of the activated methyl cycle, which recycles adenine and methionine through S-adenosylmethionine (SAM)-mediated methylation reactions, and also produces the universal quorum-sensing signal, autoinducer-2 (AI-2). SAM is also essential for synthesis of polyamines, N-acylhomoserine lactone (autoinducer-1), and production of vitamins and other biomolecules formed by SAM radical reactions. MTA, SAH and 5Ј-deoxyadenosine (5ЈdADO) are product inhibitors of these reactions, and are substrates of MTA/SAH nucleosidase, underscoring its importance in a wide array of metabolic reactions. Inhibition of this enzyme by certain substrate analogues also limits synthesis of autoinducers and hence causes reduction in biofilm formation and may attenuate virulence. Interestingly, the inhibitors of MTA/SAH nucleosidase are very effective against the Lyme disease causing spirochaete, Borrelia burgdorferi, which uniquely expresses three homologous functional enzymes. These results indicate that inhibition of this enzyme can affect growth of different bacteria by affecting different mechanisms. Therefore, new inhibitors are currently being explored for development of potential novel broad-spectrum antimicrobials.

Trust in science and scientists can greatly influence consideration of scientific developments and activities. Yet, trust is a nebulous construct based on emotions, knowledge, beliefs, and relationships. As we explored the literature regarding trust in science and scientists we discovered that no instruments were available to assess the construct, and therefore, we developed one. Using a process of data collection from science faculty members and undergraduate students, field testing, expert feedback, and an iterative process of design, we developed, validated, and established the reliability of the Trust in Science and Scientist Inventory. Our 21‐item instrument has a reliability of Cronbach's alpha of .86, and we have successfully field‐tested it with a range of undergraduate college students. We discuss implications and possible applications of the instrument, and include it in the appendix.

The enzyme 5Ј-methylthioadenosine (MTA) 1 /S-adenosylhomocysteine (AdoHcy) nucleosidase (EC 3.2.2.9) is a dual substrate specific enzyme that irreversibly cleaves the N 9 -C 1Ј bond of 5Ј-methylthioadenosine and S-adenosylhomocysteine to form adenine, and 5Ј-methylthioribose and S-ribosylhomocysteine, respectively (1). MTA/AdoHcy nucleosidase has been described as an excellent target for broad-spectrum antimicrobial drug design (2, 3). The enzyme is not found in mammalian cells but is found in many microbial pathogens, such as Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, Haemophilus influenza, Vibrio cholerae, and Bacillus anthracis. Inhibition of MTA/AdoHcy nucleosidase should selectively target the pathogenic microbes while leaving the human host unharmed by increasing cellular levels of MTA and AdoHcy. The buildup of these molecules will affect four major cellular functions in microbes. First, the enzyme is important in the recycling of methionine, an essential amino acid that is energetically expensive to synthesize (2, 3). Second, the nucleosidase is involved in the regulation of biological methylation, as AdoHcy is a potent negative feedback inhibitor of methyltransferases (4). Biological methylation is critical in all organisms and is responsible for regulating a number of biological processes including DNA and protein metabolism (5). Third, MTA/ AdoHcy nucleosidase participates in the regulation of polyamine biosynthesis (6, 7). The role of polyamines is not well understood but they are thought to be important in growth processes. MTA acts as a potent negative feedback inhibitor of spermidine synthase. Finally, the nucleosidase has recently been implicated in the quorum sensing pathway in bacteria (8). Quorum sensing is the phenomenon whereby the accumulation of small exported organic molecules called autoinducers enables a bacterial cell to sense the population of bacteria. This phenomenon was first described in the bioluminescent marine bacterium Vibrio fischeri (9 -11) and has been implicated in the regulation of virulence factors (12-14) and biofilm formation in many bacteria (15). When a threshold level of autoinducers is reached, bacteria trigger a signal transduction cascade that can alter gene expression (16). MTA/AdoHcy nucleosidase cleaves AdoHcy to generate S-ribosylhomocysteine and adenine. The enzyme LuxS acts on S-ribosylhomocysteine to form the autoinducer-2 furanone and homocysteine (8, 17). Autoinducer-2 has been implicated in intra-and interspecies communication. Inhibition of the nucleosidase is thought to prevent the formation of the product (S-ribosylhomocysteine) needed for LuxS to catalyze the production of autoinducer-2.Previously we determined the 1.9-Å resolution crystal structure of MTA/AdoHcy nucleosidase complexed with adenine * This work was supported in part by Canadian Institutes for Health Research Grant 43998 (to P. L. H.) and a doctoral research award (to J. E. L.). Station X8-C was supported by the United States Department of Energy and ...

The nucleosides, 5Ј-methylthioadenosine (MTA) 1 and S-adenosylhomocysteine (SAH) are molecules involved in key cellular functions such as biological methylation (1), polyamine biosynthesis (2, 3), methionine recycling (4, 5), and bacterial quorum sensing (6, 7). The breakdown of these nucleosides differs in microbial and mammalian systems. In many pathogenic microbes, such as Bacillus anthracis, Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Mycobacterium tuberculosis, and Helicobacter pylori, MTA and SAH are catabolized by the dual substrate specific enzyme, MTA/SAH nucleosidase (MTAN). MTAN irreversibly cleaves the glycosidic bond of MTA or SAH to form adenine and 5-methylthioribose or S-ribosylhomocysteine, respectively (8). However, in mammalian systems the nucleosidase is not present and the breakdown of MTA and SAH requires two separate enzymes, MTA phosphorylase (MTAP) (9) and SAH hydrolase (10), respectively. Given the differences in metabolism, MTA/ SAH nucleosidase has been identified and validated as a potential target for the design of broad-spectrum antimicrobials (4, 5).The structures of MTAN complexed with adenine (MTANadenine) (11), formycin A (MTAN-FMA) and 5Ј-methylthiotubercidin (MTAN-MTT) (12) have been solved and a comparison of the active site architecture reveals that MTAN is most similar to . MTAP belongs to the NP-I class of purine nucleoside phosphorylases (PNP) (14), which are proposed to * This work was supported in part by research Grant 43998 from the Canadian Institute for Health Research (CIHR), the National Institutes of Health, a New Zealand Foundation for Research, Science and Technology contract, United States Department of Agriculture Grant 02-0047, and by the United States Veterans Affairs Medical Research Program. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ࡗ This article was selected as a Paper of the Week. The atomic coordinates and structure factors ʈʈ Recipient of a CIHR Investigator Award.1 The abbreviations used are: MTA, 5Ј-methylthioadenosine; SAH, S-adenosylhomocysteine; MTAN, 5Ј-methylthioadenosine/S-adenosylhomocysteine nucleosidase; MTAP, 5Ј-methylthioadenosine phosphorylase; MT-ImmA, (1S)-1-(9-deazaadenin-9-yl)-1,4-dideoxy-1,4-imino-5-methylthio-D-ribitol; MT-DADMe-ImmA, (3R,4S)-1-[9-deazaadenin-9-yl)methyl]-3-hydroxy-4-(methylthiomethyl)pyrrolidine; MTT, 5Ј-methylthiotubercidin; PNP, purine nucleoside phosphorylase; FMA, formycin A; r.m.s.d., root mean square deviation.