Kinetic studies of a soluble alpha beta complex of nitrate reductase A from Escherichia coli. Use of various alpha beta mutants with altered beta subunits

A soluble alpha beta complex of nitrate reductase can be obtained from a strain of Escherichia coli that lacks the narI gene and expresses only the alpha and beta subunits. The beta subunit contains four Fe-S centres and the alpha subunit contains the molybdenum cofactor, which is the site at which nitrate is reduced. Despite the lack of the gamma subunit of the complete enzyme, this complex can still catalyse the reduction of nitrate with artificial electron donors such as benzyl viologen, so that it is suitable for studying the transfer of electrons between these two types of redox centre. To examine whether the electrons from reduced benzyl viologen are initially delivered to the Fe-S centres, or directly to the molybdenum cofactor, or both, we have studied the steady-state kinetics and the binding of benzyl viologen to the alpha beta complex and mutants alpha beta* with altered beta subunits. Reduction of the enzyme by reduced benzyl viologen in the absence of nitrate showed that all four Fe-S centres and the molybdenum cofactor could be reduced. Two classes of site with different equilibrium constants could be distinguished. The kinetic results suggest that benzyl viologen supplies its electrons directly to the molybdenum cofactor, at a rate showing a hyperbolic dependence on the square of the concentration of the electron donor. A reaction mechanism is proposed for the reduction of nitrate catalysed by the alpha beta complex of nitrate reductase with artificial electron donors.     

Spatial learning deficits in mice with a targeted glucocorticoid receptor gene disruption

Previous studies in rats using the Morris water maze suggested that the processing of spatial information is modulated by corticosteroid hormones through mineralocorticoid and glucocorticoid receptors in the hippocampus. Mineralocorticoid receptors appear to be involved in the modulation of explorative behaviour, while additional activation of glucocorticoid receptors facilitates the storage of information. In the present study we used the water maze task to examine spatial learning and memory in mice homozygous and heterozygous for a targeted disruption of the glucocorticoid receptor gene. Compared with wild-type controls, homozygous and heterozygous mice were impaired in the processing of spatial but not visual information. Homozygous mutants performed variably during training, without specific platform-directed search strategies. The spatial learning disability was partly compensated for by increased motor activity. The deficits were indicative of a dysfunction of glucocorticoid receptors as well as of mineralocorticoid receptors. Although the heterozygous mice performed similarly to wild-type mice with respect to latency to find the platform, their strategy was more similar to that of the homozygous mice. Glucocorticoid receptor-related long-term spatial memory was impaired. The increased behavioural reactivity of the heterozygous mice in the open field points to a more prominent mineralocorticoid receptor-mediated function. The findings indicate that (i) the glucocorticoid receptor is of critical importance for the control of spatial behavioural functions, and (ii) mineralocorticoid receptor-mediated effects on this behaviour require interaction with functional glucocorticoid receptors. Until the development of site-specific, inducible glucocorticoid receptor mutants, glucocorticoid receptor-knockout mice present the only animal model for the study of corticosteroid-mediated effects in the complete absence of a functional receptor.     

THE EARLY TO "MIDDLE" CRETACEOUS LITHOSTRATIGRAPHY OF THE CENTRAL NORTH SEA (UK SECTOR)

A revised lithostratigraphical framework of the Early to "Middle" Cretaceous strata of the Central North Sea (UK sector) derived from the rigorous examination of more than 100 commercial boreholes is presented. previously published frameworks have tended toward oversimplification of the Early to "Middle" Cretaceous, as the interval was considered initially to be of minor importance with regard to prospectivity. However_: there is a general feeling in the industry that the Early to "Middle" Cretaceous is prospective. particularly with regard to subtle stratigraphical traps like the Kilda/Bosun and Scapa fields. It is inevitable that as more attention, due to hydrocarbon exploration, is paid to an interval that the stratigraphical models used progress from the simplistic to the complex. The framework presented here is the result of a pragmatic approach in that it is based upon the integration of previously published schemes. with the erection of new units derived from new analyses and interpretation. The complete lithostratigraphical framework is defined. explained and discussed with reference to borehole examples in the Central North Sea .     

Transfer of a discrimination by pigeons (Columba livia) between pictured locations and the represented environments.

In Exp 1, pigeons (Columba livia) were trained in a successive slide presentation procedure to discriminate between pictures of 2 ends of a room and then trained to find food in the actual room. A congruent-transfer (CT) group learned the spatial discrimination more quickly than an incongruent-transfer (IT) group. In Exp 2's replication we used a simultaneous slide presentation procedure and added a control group. The IT group required significantly more trials than the CT or control groups. In Exp 3, order of the training conditions was reversed. CT and IT conditions had no effect on the speed of acquisition of the discrimination. This indicates that pigeons acquire a representation of spatial locations from pictures, which can then direct behavior, but the direction of transfer observed was unidirectional. This suggests that a discrimination between spatial locations may not be accurately represented in pictorial form. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Surface electronic structure of rare earth metals

Angle-resolved UV photoemission has been used to investigate the electronic structure of the (0001) surfaces of scandium, yttrium, praseodymium and gadolinium. Off-normal emission spectra were recorded with high angular resolution, enabling detailed mapping of the dispersion of valence band features. Yttrium and gadolinium show similar results to published data from Ho(0001), suggesting minimal 4f influence in the lanthanide bandstructures. Differences seen on praseodymium and scandium may be due to 4f derived states and surface states respectively.