Dairy industry fermentative processes mostly use Lactococcus lactis as a starter. However, some dairy L. lactis strains produce putrescine - a biogenic amine that raises food safety and spoilage concerns - via the agmatine deiminase pathway (AGDI). The enzymatic activities responsible for putrescine biosynthesis in this bacterium are encoded by the AGDI gene-cluster. The role of the catabolic genes aguB, aguD, aguA and aguC has been studied, but knowledge regarding the role of aguR (the first gene in the cluster) remains limited. In the present work, aguR was 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 the aguB promoter (PaguB), which drives transcription of the aguBDAC operon. Inverted sequences required for PaguB activity were identified by deletion analysis. Further work indicated AguR to be 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 the aguBDAC operon through a LuxR_C_like cytoplasmic DNA binding domain.