The role of subunit composition in determining intrinsic maximum activation and deactivation kinetics of GABA(A) receptor channels is unknown. We used rapid ligand application (100-micros solution exchange) to examine the effects of alpha-subunit composition on GABA-evoked activation and deactivation rates. HEK 293 cells were transfected with human cDNAs encoding alpha1beta1gamma2- or alpha2beta1gamma2-subunits. Channel kinetics were similar across different transfections of the same subunits and reproducible across several GABA applications in the same patch. Current rise to peak was at least twice as fast for alpha2beta1gamma2 receptors than for alpha1beta1gamma2 receptors (reflected in 10-90% rise times of 0.5 versus 1.0 ms, respectively), and deactivation was six to seven times slower (long time constants of 208 ms versus 31 ms) after saturating GABA applications. Thus alpha-subunit composition determined activation and deactivation kinetics of GABA(A) receptor channels and is therefore likely to influence the kinetics and efficacy of inhibitory postsynaptic currents.

Synthesis of cobalamin de novo by Salmonella enterica serovar Typhimurium strain LT2 and the absence of this ability in Escherichia coli present several problems. This large synthetic pathway is shared by virtually all salmonellae and must be maintained by selection, yet no conditions are known under which growth depends on endogenous B 12 . The cofactor is required for degradation of 1,2-propanediol and ethanolamine. However, cofactor synthesis occurs only anaerobically, and neither of these carbon sources supports anaerobic growth with any of the alternative electron acceptors tested thus far. This paradox is resolved by the electron acceptor tetrathionate, which allows Salmonella to grow anaerobically on ethanolamine or 1,2-propanediol by using endogenously synthesized B 12 . Tetrathionate provides the only known conditions under which simple cob mutants (unable to make B 12 ) show a growth defect. Genes involved in this metabolism include the ttr operon, which encodes tetrathionate reductase. This operon is globally regulated by OxrA (Fnr) and induced anaerobically by a two-component system in response to tetrathionate. Salmonella reduces tetrathionate to thiosulfate, which it can further reduce to H 2 S, by using enzymes encoded by the genes phs and asr. The genes for 1,2-propanediol degradation (pdu) and B 12 synthesis (cob), along with the genes for sulfur reduction (ttr, phs, and asr), constitute more than 1% of the Salmonella genome and are all absent from E. coli. In diverging from E. coli, Salmonella acquired some of these genes unilaterally and maintained others that are ancestral but have been lost from the E. coli lineage.Virtually all Salmonella isolates synthesize B 12 de novo under anaerobic conditions (27,34,43). The ability to synthesize and import B 12 requires more than 35 known genes (48)-approaching 1% of the genome. However, mutations that eliminate B 12 synthesis from otherwise wild-type strains cause no growth defect under the standard aerobic or anaerobic lab conditions used thus far. Since evolutionary maintenance of such a large fraction of the genome requires selection, it seems inescapable that natural conditions must exist under which endogenously synthesized B 12 is important to growth of salmonellae. Salmonella enterica serovar Typhimurium makes B 12 de novo only in the absence of oxygen (27). Degradation of ethanolamine or 1,2-propanediol requires B 12 and provides a carbon and energy source, but growth on these compounds has been observed only under aerobic conditions requiring exogenous B 12 (28,46). These paradoxical aspects of B 12 metabolism have been reviewed (47).The B 12 paradox may be resolved by the finding, described here, that the electron acceptor tetrathionate supports anaerobic use of ethanolamine or 1,2-propanediol as the sole carbon and energy source by using endogenously synthesized B 12 . Under anaerobic conditions, tetrathionate supports considerably better growth on these carbon sources than the other alternative electron acceptors tested. Tetrathionate plu...