The gene encoding an extracellular levan fructotransferase, designated the lft gene, was cloned from the genomic DNA of Arthrobacter nicotinovorans GS-9, and expressed in Escherichia coli. It was found that a single open reading frame consisted of 1554 base pairs that encoded a polypeptide composed of a signal peptide of 33 amino acids and a mature protein of 484 amino acids (M_r 53, 152), and it was also found that a putative ribosome-binding site was present in the upstream from the ORF. The primary structure had no significant similarity with those of inulin fructotransferases, but had low similarity to the catalytic regions of other fructosylhydrolases. The expression of the lft gene was increased on a plasmid, pLFT-BB1, in which the lft gene was fused with α-peptide of the lacZ gene of pUC18. An E. coli transformant carrying pLFT-BB1 expressed six times as much activity of levan fructotransferase as that of the original strain, A. nicotinovorans GS-9.

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