A constitutive unregulated expression of β-galactosidase in Lactobacillus fermentum M1.

A constitutively β-galactosidase (LacL)-producing Lactobacillus fermentum M1 isolated from fermented milk was found to produce β-galactosidase in the presence of glucose. β-galactosidase activity produced in glucose (30 mM) medium was 2.17 U/mL as compared to 2.27 and 2.19 U/mL with galactose and lactose, respectively. When a combination of glucose (30 or 60 mM) with galactose (30 mM) was used as carbon source, β-galactosidase activity was not repressed rather was found increased when compared to carbon sources used individually. In real-time PCR analysis of mRNA synthesized on individual and combined carbon sources, repression of the lacL gene expression was not observed. This observation suggests that the strain M1 lacked normal carbon catabolite repression. Examination of nucleotide sequence of lacL identified two catabolite responsive elements (cre): cre1 located downstream near the promoter region and cre2 within the coding sequence. Each of which differed from the 14-bp consensus by a single nucleotide. In cre1, it is C in place of highly conserved T at position 1 in the consensus. In cre 2, it is G in place of C, a residue completely conserved at position 13. Since catabolite genes in Gram-positive bacteria are regulated by carbon catabolite protein A (CcpA) through interaction with DNA at a specific cis-acting cre, it is assumed that base changes at conserved position in the cre elements disrupt CcpA binding and thereby leading to constitutive expression of lacL gene. The study noted to be the first report about the constitutive production of β-galactosidase in L. fermentum.