The present invention relates to an isolated polynucleotide comprising an open reading frame encoding a polypeptide having alpha-amylase activity, the polypeptide selected from the group consisting of: a) a polypeptide comprising an amino acid sequence which has at least 70% identity with amino acids 22 to 450 of SEQ ID NO: 4; b) a polypeptide comprising an amino acid sequence which has at least 70% identity with the polypeptide encoded by the amylase encoding part of the polynucleotide inserted into a plasmid present in the E. coli host deposited under the Budapest Treaty with DSMZ under accession number DSM 15334; c) a polypeptide encoded by a polynucleotide comprising a nucleotide sequence which has at least 70% identity with the sequence shown from position 68 to 1417 in SEQ ID NO: 3; and d) a fragment of (a), (b) or (c) that has alpha-amylase activity.