LAUSR

L-Carnitine is essential in the intermediary metabolism of eukaryotes and involved in β-oxidation of medium- and long-chain fatty acids, thus, it has applications for medicinal purposes and as dietary supplement. In addition, L-carnitine plays roles in bacterial physiology and metabolism, which has been used by industry to develop biotechnological carnitine production processes. Here, based on studies of L-carnitine metabolism in Escherichia coli and its activation by the transcriptional activator CaiF a biosensor was developed that expressed a fluorescent reporter gene responding in a dose dependent manner to crotonobetainyl-CoA, an intermediate of L-carnitine metabolism in E. coli, which is proposed to be the co-activator of CaiF. Moreover, a dual input biosensor for L-carnitine and crotonobetaine was developed. As application, potential homologues of betaine:CoA ligase CaiC from Citrobacter freundii, Proteus mirabilis, and Arcobacter marinus were screened and shown to be functionally active CaiC variants. These and the developed biosensor may be valuable for improving L-carnitine production processes.