Self-replicating chemical systems have been designed and studied to identify the minimal requirements for molecular replication, to translate the principle into synthetic supramolecular systems and to derive a better understanding of the scope and limitations of self-organization processes that are believed to be relevant to the origin of life on Earth. Current implementations make use of oligonucleotide analogues, peptides, and other molecules as templates and are based either on autocatalytic, cross-catalytic, or collectively catalytic pathways for template formation. A common problem of these systems is product inhibition, leading to parabolic instead of exponential amplification. The latter is the dynamic prerequisite for selection in the darwinian sense. We here describe an iterative, stepwise procedure for chemical replication which permits an exponential increase in the concentration of oligonucleotide analogues. The procedure employs the surface of a solid support and is called SPREAD (surface-promoted replication and exponential amplification of DNA analogues). Copies are synthesized from precursor fragments by chemical ligation on immobilized templates, and then liberated and immobilized to become new templates. The process is repeated iteratively. The role of the support is to separate complementary templates which would form stable duplexes in solution. SPREAD combines the advantages of solid-phase chemistry with chemical replication, and can be further developed for the non-enzymatic and enzymatic amplification of RNA, peptides and other templates as well as for studies of in vitro evolution and competition in artificial chemical systems. Similar processes may also have played a role in the origin of life on Earth, because the earliest replication systems may have proliferated by spreading on mineral surfaces.
Several bacterial species are adapted to nicotine, the main alkaloid produced by the tobacco plant, as growth substrate. A general outline of nicotine catabolism by these bacteria is presented, followed by an emphasis on new insights based on molecular biology and biochemical work obtained with the catabolic plasmid pAO1 of Arthrobacter nicotinovorans. Its 165-kb sequence revealed the genetic structure of nicotine catabolism and allowed the assignment of new enzyme activities to specific gene products, which extends the known biochemical steps of this pathway. Potential implications of the progress in our understanding of bacterial breakdown of nicotine for biotechnological applications are discussed.
The genes of nicotine dehydrogenase (NDH) were identified, cloned and sequenced from the catabolic plasmid pAO1 of Arthrobacter nicotinovorans. In immediate proximity to this gene cluster is the beginning of the 6-hydroxy-L-niotine oxidase (6-HLNO) gene. NDH is composed of three subunits (A, B and C) of M(r) 30,011, 14,924 and 87,677. It belongs to a family of bacterial hydroxylases with a similar subunit structure; they have molybdopterin dinucleotide, FAD and Fe-S clusters as cofactors. Here the first complete primary structure of a bacterial hydroxylase is provided. Sequence alignments of each of the NDH subunits show similarities to the sequences of eukaryotic xanthine dehydrogenase (XDH) but not to other known molybdenum-containing bacterial enzymes. Based on alignment with XDH it is inferred that the smallest subunit (NDHB) carries an iron-sulphur cluster, that the middle-sized subunit (NDHA) binds FAD, and that the largest NDH subunit (NDHC) corresponds to the molybdopterin-binding domain of XDH. Expression of both the ndh and the 6-hino genes required the presence of nicotine and molybdenum in the culture medium. Tungsten inhibited enzyme activity but not the synthesis of the enzyme protein. The enzyme was found in A. nicotinovorans cells in a soluble form and in a membrane-associated form. In the presence of tungsten the fraction of membrane-associated NDH increased.
The gram-positive soil bacterium Arthrobacter nicotinovorans (formerly known as Arthrobacter oxidans and reclassified by Kodama et al. [22]) has the ability to use nicotine as its sole carbon and energy source (8,10,15,17). Nicotine, the alkaloid of the tobacco plant, is synthesized as the L-isomer, and the first enzyme to attack L-nicotine is a trimeric, molybdopterin cofactor (MoCo) (most probably in its dinucleotide form [18])-, flavin adenine dinucleotide (FAD)-, and [Fe-S] clustercontaining nicotine dehydrogenase (NDH), which hydroxylates the pyridine ring at position 6 (Fig.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.