This study aimed to characterize experimentally the biochemical activity of the TunG protein using heterologous expression in Escherichia coli (E. coli). The tunG gene was codon-optimized, cloned into a T7 promoter-based expression vector, and expressed in E. coli BL21(DE3) cells. The TunG protein was purified via nickel-affinity chromatography and analyzed by SDS-PAGE. This confirmed successful expression but revealed co-purification of endogenous E. coli proteins. Enzymatic activity was assessed using High-Performance Liquid Chromatography (HPLC) to monitor reactions with a range of nucleotide substrates, including uridine monophosphate (UMP), adenosine monophosphate (AMP), thymidine monophosphate (TMP), and higher-order uridine nucleotides under varying reaction conditions.

HPLC analyses revealed nucleotide turnover across multiple substrates; however, comparable activity was observed in both TunG-expressing and control samples lacking the tunG gene. Metal ion dependence, substrate specificity, and reaction condition profiles were consistent with known E. coli nucleotidases, particularly YfbR and SurE, rather than a TunG-specific activity. Diphosphate and triphosphate substrates, including UDP, UTP, and UDP-GlcNAc, remained largely unprocessed after extended incubation, further suggesting that the endogenous E. coli enzymes do not function as a general nucleotide phosphatase.

Collectively, these results indicate that the observed enzymatic activity arises from endogenous E. coli enzymes rather than the TunG protein itself. However, this work narrows the range of plausible biochemical functions for TunG and highlights the challenges of characterizing pathway-specific enzymes outside their native biosynthetic context. Future studies will focus on improved purification, alternative substrates, and pathway reconstitution will be essential to fully define the role of the TunG protein in tunicamycin biosynthesis.