Bacillus subtilis is capable of degrading fructosamines. The phosphorylation and the cleavage of the resulting fructosamine 6-phosphates is catalyzed by the frlD and frlB gene products, respectively. This study addresses the physiological importance of the frlBONMD genes (formerly yurPONML), revealing the necessity of their expression for growth on fructosamines and focusing on the complex regulation of the corresponding transcription unit. In addition to the known regulation by the global transcriptional regulator CodY, the frl genes are repressed by the convergently transcribed FrlR (formerly YurK). The latter causes repression during growth on substrates other than fructosamines. Additionally, we identified in the first intergenic region of the operon an FrlR binding site which is centrally located within a 38-bp perfect palindromic sequence. There is genetic evidence that this sequence, in combination with FrlR, contributes to the remarkable decrease in the transcription downstream of the first gene of the frl operon.Amadori products (fructosamines), the first stable intermediates of the Maillard reaction, result from nonenzymatic glycation of amino acids or proteins with reducing sugars such as glucose. As Amadori products are found in heated food and cause several diseases in connection with diabetes and aging (4), enzymes catalyzing the degradation of Amadori products are of interest to industry and medicine for food processing and diagnostic purposes.There are two enzymatic mechanisms for the deglycation of Amadori products (reviewed in reference 7): fructosyl-amino acid oxidases (known as Amadoriases) and fructosamine kinases. The former deglycate by means of oxidation, generating the corresponding amino acid, glucosone, and H 2 O 2 . Amadoriases have been found in Aspergillus and Penicillium spp. (38, 42) but also in bacteria such as Arthrobacter and Corynebacterium spp. (9, 32). Fructosamine kinases phosphorylate the Amadori products prior to cleavage. Mammalian enzymes add the phosphate at C-3, and the product subsequently undergoes autocatalytic degradation (20). However, Escherichia coli and Bacillus subtilis kinases phosphorylate at C-6; further processing of such intermediates needs a second deglycase enzyme catalyzing the cleavage of the fructosamine 6-phosphates to generate, for example, glucose 6-phosphate and a free amine.E. coli grows on fructosyl-lysine as the sole carbon and nitrogen source, and expression of frlD (encoding the kinase) and frlB (encoding the deglycating protein) is induced by fructosyl-lysine (39). In B. subtilis, frlD and frlB belong to a gene cluster which additionally comprises three genes coding for putative transporters (frlONM) as well as the convergently oriented putative repressor gene frlR (Fig. 1). Substrate specificities of FrlD and FrlB are quite different than those of the enzymes of E. coli, as B. subtilis acts on ␣-glycated amino acids rather than on ε-glycated lysine (catalytic efficiencies Ͼ30-fold higher), the latter being the preferred substrate of the E...

Amadori products (fructosamines)-ubiquitously occurring in nature-are precursors of the toxic and cell damaging 'advanced glycation endproducts'; thus, it is not surprising that numerous organisms have developed systems to degrade such compounds. The deglycating enzymes differ with respect to their mechanisms as well as to their substrate specificities. Furthermore, different physiological functions are proposed for the different enzymes. The fructosamine 3-kinases of mammals and homologous proteins (fructosamine 3-kinase related proteins), which are common to all taxa, are thought to focus on intracellular repair functions. In contrast, in Bacillus subtilis and Escherichia coli, the cooperative action of a kinase and a deglycase facilitates Amadori degradation. As genes encoding these enzymes are co-transcribed with ABC transporter genes, it is thought that these genes facilitate the utilisation of extracellular Amadori products. Indeed, it has been shown that fructosamines can serve as the sole carbon and nitrogen sources. Here, we provide an overview of known deglycating systems with the emphasis on Amadori product degradation in bacteria.

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