SUMMARY: The gene from A054, a commercial yogurt strain, was cloned on a 7·2 kb I fragment in and compared with the previously cloned gene from ATCC 19258. Using the dideoxy chain termination method, the DNA sequences of both structural genes were determined and found to be 3071 bp in length. When the two sequences were more closely analysed, 21 nucleotide differences were detected, of which only nine resulted in amino acid changes in the proteins, the remainder occurring in wobble positions of the respective codons. Only three bases separated the termination codon for the gene from the initiation codon for , suggesting that the lactose utilization genes are organized as an operon. The amino acid sequence of the β-galactosidase, derived from the DNA sequence, corresponds to a protein with a molecular mass of 116860 Da. Comparison of the amino acid sequences with those from and showed 48, 35 and 32·5% identity respectively. Although little sequence homology was observed at the DNA level, many regions conserved in the amino acid squence were identified when the β-galactosidase proteins from and were compared.


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