Insights into lactose catabolism and its regulation in Actinobacillus succinogenes

Actinobacillus succinogenes 130Z is a Gram-negative bacterium, with capacity to produce high concentrations of succinic acid through fermentation of carbohydrates, including lactose. This study investigated lactose metabolism through growth analyses, enzyme assays, and transcriptomics and proteomics approaches.

Actinobacillus succinogenes showed efficient growth on lactose with only a 15% reduction when compared to glucose, while growth on galactose was significantly slower (75% reduction). Enzyme assays revealed that β-galactosidase activity is cytoplasmic and induced 13-fold during growth on lactose. A strain with a deletion of gene Asuc_1398 (encoding a β-galactosidase) was unable to grow on lactose, confirming β-galactosidase essential role. Transcriptomics and proteomics analyses identified 39 transcripts and 94 proteins displaying differential expression when comparing lactose to glucose as carbon source. Lactose-dependent upregulation was detected for genes Asuc_1396 (encoding LacI), Asuc_1397 (encoding LacY), and Asuc_1398 (encoding LacZ). Additional upregulated genes encode functions involved in maltose transport/metabolism, DNA uptake/transformation, and mannose transport. Interestingly, galactose metabolism genes from the Leloir pathway showed no differential expression. Bioinformatic analyses identified putative operon organization of several genes, and CRP-like and LacI-like DNA binding sites upstream of differentially expressed genes.

Growth on lactose caused the differential expression of proteins involved in the catabolism of this sugar, as well as proteins encoding functions associated with a state identified as carbon limitation in several bacterial species. Our findings suggest that genes Asuc_1397-1398 form an operon induced in the presence of lactose and contains CRP-like and LacI-like binding sites in the promoter regions.