ABSTRACTDairy industry fermentative processes mostly useLactococcus lactisas a starter. However, some dairyL. lactisstrains produce putrescine, a biogenic amine that raises food safety and spoilage concerns, via the agmatine deiminase (AGDI) pathway. The enzymatic activities responsible for putrescine biosynthesis in this bacterium are encoded by the AGDI gene cluster. The role of the catabolic genesaguB,aguD,aguA, andaguChas been studied, but knowledge regarding the role ofaguR(the first gene in the cluster) remains limited. In the present work,aguRwas found to be a very low level constitutively expressed gene that is essential for putrescine biosynthesis and is transcribed independently of the polycistronic mRNA encoding the catabolic genes (aguBDAC). In response to agmatine, AguR acts as a transcriptional activator of theaguBpromoter (PaguB), which drives the transcription of theaguBDACoperon. Inverted sequences required for PaguBactivity were identified by deletion analysis. Further work indicated that AguR is a transmembrane protein which might function as a one-component signal transduction system that senses the agmatine concentration of the medium and, accordingly, regulates the transcription of theaguBDACoperon through a C-terminal cytoplasmic DNA-binding domain typically found in LuxR-like proteins.