Asn-346 replacement reduced significantly the MICs of all β-lactams, except the Asn-346-Ile substitution that increased the MICs of cephalosporins, whereas it decreased those of carbapenems. The biochemical characterization, along with a molecular modeling study, showed that the size and the polarity of the side chain at position 346 assisted substrate binding and turnover. This study shows for the first time that the amino acid at position 346 contributes to the β-lactamase activity of cephalosporinases. Asparagine and isoleucine residues, which are well conserved
at position 346 among AmpC-type enzymes, modulate their hydrolysis spectrum in an opposing sense. Ile-346 Gefitinib confers higher level of cephalosporins resistance, whereas Asn-346 confers carbapenem resistance in combination with outer membrane impermeability. “
“Inhibition by light potentially influences the distribution of ammonia oxidizers in aquatic environments and is one explanation for nitrite maxima near the base of the euphotic zone
of oceanic waters. Previous studies of photoinhibition have been restricted to bacterial ammonia oxidizers, rather than archaeal ammonia oxidizers, which dominate in marine environments. To compare the photoinhibition of bacterial and archaeal ammonia oxidizers, specific growth rates of two ammonia-oxidizing archaea (Nitrosopumilus maritimus and Nitrosotalea devanaterra) and bacteria (Nitrosomonas europaea and Nitrosospira multiformis) were determined at different light intensities under continuous illumination and light/dark NU7441 price cycles. All strains were inhibited by continuous illumination at the highest intensity (500 μE m−2 s−1). At lower light intensities, archaeal growth was much more photosensitive than bacterial growth, with greater inhibition at 60 μE m−2 s−1 than at 15 μE m−2 s−1, where bacteria were unaffected. Archaeal ammonia oxidizers were also more sensitive to cycles of 8-h light/16-h darkness at two light intensities
(60 and 15 μE m−2 s−1) and, unlike bacterial strains, showed no evidence of recovery during dark phases. The findings provide evidence for niche differentiation in aquatic environments and reduce support for photoinhibition as an explanation SB-3CT of nitrite maxima in the ocean. Nitrification is a key process in the cycling of nitrogen in terrestrial and aquatic ecosystems. The first, rate-limiting step of nitrification, the oxidation of ammonia (NH3) to nitrite (), is carried out by both ammonia-oxidizing bacteria (AOB, Koops & Pommerening-Röser, 2001) and archaea belonging to the recently described thaumarchaea group (AOA, Spang et al., 2010). The first step in ammonia oxidation is catalysed by ammonia monooxygenase, and the subunit A gene (amoA) is the most commonly used marker for tracking ammonia oxidizers in environmental samples.