α-Amylase of Clostridium thermosulfurogenes EM1: Nucleotide sequence of the gene, processing of the enzyme, and comparison to other α-amylases

Abstract

The nucleotide sequence of the α-amylase gene (amyA) from Clostridium thermosulfurogenes EM1 cloned in Escherichia coli was determined. The reading frame of the gene consisted of 2,121 bp. Comparison of the DNA sequence data with the amino acid sequence of the N terminus of the purified secreted protein of C. thermosulfurogenes EM1 suggested that the α-amylase is translated from mRNA as a secretory precursor with a signal peptide of 27 amino acid residues. The deduced amino acid sequence of the mature α-amylase contained 679 residues, resulting in a protein with a molecular mass of 75,112 Da. In E. coli the enzyme was transported to the periplasmic space and the signal peptide was cleaved at exactly the same site between two alanine residues. Comparison of the amino acid sequence of the C. thermosulfurogenes EM1 α-amylase with those from other bacterial and eucaryotic α-amylases showed several homologous regions, probably in the enzymatically functioning regions. The tentative Ca2+-binding site (consensus region I) of this Ca2+-independent enzyme showed only limited homology. The deduced amino acid sequence of a second obviously truncated open reading frame showed significant homology to the malG gene product of E. coli. Comparison of the α-amylase gene region of C. thermosulfurogenes EM1 (DSM3896) with the β-amylase gene region of C. thermosulfurogenes (ATCC 33743) indicated that both genes have been exchanged with each other at identical sites in the chromosomes of these strains.