The commercially important bacterium Lactococcus lactis contains two FNR-like proteins (FlpA and FlpB) which have a high degree of identity to each other and to the FLP of Lactobacillus casei. FlpA was isolated from a GST-FlpA fusion protein produced in Escherichia coli. Like FLP, isolated FlpA is a homodimeric protein containing both Zn and Cu. However, the properties of FlpA were more like those of the E. coli oxygen-responsive transcription factor FNR than the FLP of L. casei. As prepared FlpA recognized an FNR site (TTGAT-N4-ATCAA) but not an FLP site (CCTGA-N4-TCAGG) in band-shift assays. In contrast to FLP, DNA binding by FlpA did not require the formation of an intramolecular disulphide bond. However, despite containing only two cysteine residues per monomer, FlpA was able to acquire an FNR-like, oxygen-labile [4Fe 4S] cluster. But, whereas the incorporation of a [4Fe 4S] cluster into FNR enhances interaction with target DNA, it abolished DNA binding by FlpA. An FlpA variant (FlpA') with an N-terminal region designed to be more FLP-like failed to incorporate an iron-sulphur cluster but could now form an intramolecular disulphide. This simple example of protein engineering, converting an oxygen-labile [4Fe 4S] containing FNR-like protein into a dithiol-disulphide FLP-like redox sensor demonstrates the versatility of the basic CRP structure. Attempts to demonstrate an FlpA-based aerobic-anaerobic switch in the heterologous host E. coli were unsuccessful. However, studies with a series of FNR-dependent lac reporter fusions in strains of E. coli expressing flpA or flpB revealed that both homologues were able to activate expression of FNR-dependent promoters in vivo but only when positioned 61 base pairs upstream of the transcription start.