Stable isotopic signature of dissimilatory nitrate reduction is robust against enzyme mutation

• 成果类型: Article
• 署名作者: Asamoto, Ciara K.; Ryu, Yeongjun; Eckartt, Kelly N.; Kelley-Kern, Julia; Dietrich, Lars E. P.; Sigman, Daniel M.; Kopf, Sebastian H.
• 署名单位: University of Colorado System; University of Colorado Boulder; Princeton University; Columbia University
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-9038
• DOI: 10.1073/pnas.2416002121
• 发表日期: 2024-11-26
• 关键词: fractionation; denitrification; nitrogen

摘要

The proportionality of oxygen-to-nitrogen isotope effects ((18)epsilon/(15)epsilon) is used as a key isotopic signature of nitrogen cycling processes in the environment. Dissimilatory nitrate reduction is observed to have an (18)epsilon/(15)epsilon proportionality of similar to 0.9 in marine and similar to 0.6 in freshwater/terrestrial ecosystems. The origins of this difference are uncertain, with both geochemical and biological factors conceivably at play. One potential factor is variation in the isotope effect of nitrate reduction among different forms of the nitrate reductase enzyme. NarG nitrate reductases are observed to typically have an (18)epsilon/(15)epsilon of similar to 0.9. However, a recent study uncovered an exception, with Bacillus NarG enzymes having an (18)epsilon/(15)epsilon proportionality of similar to 0.6. This provides an opportunity to investigate genetic controls on (18)epsilon/(15)epsilon. Furthermore, this atypical NarG signature also raises the question of whether intrinsic isotope signatures can evolve as the enzymes that produce them accumulate mutations through time. Here, we present data from site-directed mutagenesis experiments of key NarG residues, which suggest that the distinct Bacillus (18)epsilon/(15)epsilon cannot be caused by single mutations alone and is potentially uncommon in nature. Variation in the intrinsic isotope effects of an enzyme through time may thus require more extensive evolutionary changes.