Thermotoga neapolitana gene clusters participating in degradation of starch and maltodextins: molecular structure of the locus

A 5451-bp genome fragment of the hyperthermophilic anaerobic eubacterium Thermotoga neapolitana has been cloned and sequenced. The fragment contains one truncated and three complete open reading frames highly homologous to the starch/maltodextrin utilization gene cluster from Thermotoga maritima whose genome sequence is known. The incomplete product of the first frame is highly homologous to MalG, the E. coli protein of starch and maltodextrin transport. The product of the second frame, AglB, is highly homologous to cyclomaltodextrinase with the alpha-glucosidase activity TMG belonging to family 13 of glycosyl hydrolases (GH13). The product of the third frame, AglA, is homologous to the Thermotoga maritima cofactor-dependent alpha-glucosidase from the GH4 family. The two enzymes form a separate branch on the phylogenetic tree of the family. The AglA and AglB proteins supplement each other in substrate specificity and can ensure complete hydrolysis to glucose of cyclic and linear maltodextrins, the intermediate products of starch degradation. The product of the fourth reading frame has sequence similarity with the riboflavin-specific deaminase RibD from T. maritima. The homologous locus of this bacterium, between the aglA and ribD genes, has five open reading frames missing in T. neapolitana. The nucleotide sequences of two frames are homologous to transposase genes. The deletion size is 2.9 kb.