Development and retention of a primordial moonlighting pathway of protein modification in the absence of selection presents a puzzle

Significance The LipB–LipA and lipoyl-relay pathways seem restricted to gram-negative and gram-positive bacteria, respectively. Braakman and Smith have persuasively argued that the lipoyl-relay pathway involving transfer of lipoic acid from lipoylated GcvH is the primordial pathway and that the LipB–LipA pathway arose later. The chemically challenging nature of lipoyl transfer strengthens this argument. Given the roughly 3 billion years since the two groups of bacteria diverged, it is unexpected and puzzling that the lipoyl transfer properties of many GcvHs—those from eukaryotes and bacteria, including Escherichia coli , and the deep-branching hyperthermophilic chemoautotrophic bacterium, Aquifex aeolicus —have lipoyl-relay activity in Bacillus subtilis . This provides a remarkable case in which a protein “moonlighting” function is found throughout evolution without selection for the function.