The ABC superfamily is a diverse group of integral membrane proteins involved in the ATP-dependent transport of solutes across biological membranes in both prokaryotes and eukaryotes. Although ABC transporters have been studied for over 30 years, very little is known about the mechanism by which the energy of ATP hydrolysis is used to transport substrate across the membrane. The recent report of the high resolution crystal structure of HisP, the nucleotide-binding subunit of the histidine permease complex of Salmonella typhimurium, represents a significant breakthrough toward the elucidation of the mechanism of solute translocation by ABC transporters. In this review, we use data from the crystallographic structures of HisP and other nucleotide-binding proteins, combined with sequence analysis of a subset of atypical ABC transporters, to argue a new model for the dimerisation of the nucleotide-binding domains that embraces the notion that the C motif from one subunit forms part of the ATP-binding site in the opposite subunit. We incorporate this dimerisation of the ATP-binding domains into our recently reported L-barrel model for P-glycoprotein and present a general model for the cooperative interaction of the two nucleotide-binding domains and the translocation of mechanical energy to the transmembrane domains in ABC transporters. ß

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This study measured the effects of a single pulse of chlorpyrifos at nominal concentrations of 1 and 10 microg/l on the macroinvertebrate community structure of a coastal stream mesocosm system. Analysis of data using Principal Response Curves (PRC) and Monte Carlo tests showed significant changes in the treated stream mesocosms relative to that of the controls. These changes in the macroinvertebrate assemblages occurred within 6 h, and persisted for at least 124 days after dosing. Significant community-level effects were detected at the lowest concentration on days 2 and 16 post-dosing, giving a no-observed effect concentration (NOEC(community)) of 1.2 microg/l (measured). The mayflies Atalophlebia sp. and Koorrnonga sp., Chironomidae and Acarina were all sensitive to chlorpyrifos and decreased in abundance in treated mesocosms after dosing. The fauna of these coastal stream mesocosms showed similar sensitivity to chlorpyrifos with that of other reported studies, but there was no evidence of recovery after 124 days.

The hyporheic zone is an important refuge for invertebrates as surface water recedes in temporary streams. In this study, the structure and functional organisation of hyporheic macroinvertebrate assemblages in pool and dry riffle bed habitats of two episodic streams were compared over summer and winter. Multivariate analyses revealed macroinvertebrate assemblages differed significantly between streams, habitats and seasons. While some seasonal differences were expected, the differences between streams were not, given the similarity and proximity of the catchments, and were due to shifts in the abundance of common taxa. Distinct differences between riffle and pool habitats were evident in both the taxonomic and functional feeding group composition of the assemblages. In particular, riffle habitats contained greater numbers of taxa and individuals and a greater proportion of filter-feeding animals compared to pool habitats. Summer samples also had greater numbers of taxa and individuals and greater proportions of collector-scrapers than winter samples. The relative abundance of functional groups was similar between streams in summer but was more variable in winter. Patterns observed in the taxonomic and functional feeding group structure of the macroinvertebrate assemblages were more characteristic of perennial than episodic streams, despite the absence of regular surface flows. This could be attributed to the relatively constant hyporheic flow in these streams. We suggest that classifications of stream flow should consider hyporheic discharge (not just surface flow) as this clearly influences the stream biota. Hydrobiologia (2005) 532: 81-90 Ó Springer 2005

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