Nuotein

Nuotein is a proposed family of small regulatory proteins described in diverse bacteria. Members are typically 70–100 amino acids long, lack signal peptides, and are predicted to be soluble cytosolic proteins. A conserved core sequence and a compact fold support homodimerization in several characterized members, though full structures are known for only a subset.

Functionally, nuoteins are implicated in nucleotide-responsive regulation. In many genomes they sit in operons with nucleotide metabolism or energy-related genes, suggesting roles in sensing cellular energy or nucleotide pools. Biochemical data from a subset indicate possible binding of nucleotides such as ATP, ADP, NAD+/NADH, or cyclic nucleotides, with regulatory output that may affect transcription factors, enzymes, or RNA polymerase. However, the precise mechanisms vary among members, and not all show clear nucleotide binding.

Genomic distribution appears broad but patchy, with presence in several phyla and signs of horizontal transfer. The evolutionary origin and diversification of nuoteins are active areas of study, and functional annotation remains tentative for many members.

Research status: Nuoteins are a relatively new and sometimes controversial annotation, requiring experimental validation to confirm physiological roles. If validated, they could illuminate minimal nucleotide-sensing modules and inform the design of synthetic regulatory elements in microbes.