Biological nitrogen fixation is an inherent trait exclusive to a select number of prokaryotes. Although molybdenum nitrogenase is the dominant catalyst for dinitrogen reduction, some diazotrophs also contain one or two additional types of nitrogenase that use alternative metal content as the active-site cofactor. The occurrence of alternative nitrogenases has not been well studied due to the discriminatory expression of the molybdenum nitrogenase and lack of comprehensive genomic data. This study reports on the genomic analysis of 87 unique species containing alternative nitrogenase sequences. The distribution of nitrogen-fixing genes within these species from distinct taxonomic groups shows the presence of the minimum gene set required for nitrogen fixation, including catalytic and biosynthetic enzymes of the Mo-dependent system (NifHD-KENB) and the varying occurrence of additional Nif-dedicated components. These include NifS and NifU, found primarily in aerobic species, thus suggesting that these genes are necessary to accommodate the high demand for FeÀ S clusters during aerobic nitrogen fixation.
Transfer RNAs (tRNAs) are essential adaptors that mediate translation of the genetic code. These molecules undergo a variety of post-transcriptional modifications, which expand their chemical reactivity while influencing their structure, stability, and functionality. Chemical modifications to tRNA ensure translational competency and promote cellular viability. Hence, the placement and prevalence of tRNA modifications affects the efficiency of aminoacyl tRNA synthetase (aaRS) reactions, interactions with the ribosome, and transient pairing with messenger RNA (mRNA). The synthesis and abundance of tRNA modifications respond directly and indirectly to a range of environmental and nutritional factors involved in the maintenance of metabolic homeostasis. The dynamic landscape of the tRNA epitranscriptome suggests a role for tRNA modifications as markers of cellular status and regulators of translational capacity. This review discusses the non-canonical roles that tRNA modifications play in central metabolic processes and how their levels are modulated in response to a range of cellular demands.
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