Bacterial periplasmic proteins (bPBPs) undergo drastic conformational changes upon binding substrate, making them appealing as novel molecular recognition tools for biosensing. A putative bPBP-encoding gene, socA, belongs to the soc operon responsible for santhopine (fructosyl glutamine, FQ) catabolism of Agrobacterium tumefaciens. The socA gene was isolated and expressed in Escherichia coli as a soluble 28.8 kDa periplasmic protein to investigate its properties as a potential bPBP for fructosyl amino acid (FA). The auto-fluorescence of SocA was used to monitor the protein's conformational change resulting from substrate binding. The fluorescence intensity changed upon binding FQ in a concentration dependent manner with a calculated Kd of 2.1 mu M, but was unaffected by the presence of sugars or amino acid. Our results demonstrate that SocA is a novel FA bPBP that can be utilized as a novel molecular recognition element for the monitoring of FA. (c) 2005 Elsevier Inc. All rights reserved.