Universal stress proteins (USPs) are abundant and widely distributed proteins. Even so, their mode of function is hardly understood. This study focuses on UspG (UP12) of Escherichia coli, which belongs to the UspFG subfamily. Resolution of UspG by two-dimensional gel electrophoresis uncovered a posttranslational modification during its overexpression in E. coli. One isoform represented the adenylated/phosphorylated state of UspG. In vitro experiments with His-tagged UspG revealed intrinsic autophosphorylation and autoadenylation activity. Moreover, covalently bound AMP could be released from UspG by piperidine treatment and subsequent thin-layer chromatography. UspG was characterized as a dimer, a property that got lost in a C-terminal truncated UspG. Moreover, the C-terminal part was found to be important for structural stability, because the truncation of six C-terminal amino acids resulted in a protein that was further truncated by 18 amino acids in vivo. The truncated UspG was still enzymatically active, albeit the activities were significantly reduced.