Selection and properties of alpha-acetolactate decarboxylase-deficient spontaneous mutants of Streptococcus thermophilus

Improving the acidic stability of Staphylococcus aureus α-acetolactate decarboxylase in Bacillus subtilis by changing basic residues to acidic residues

The α-acetolactate decarboxylase (ALDC) can reduce diacetyl fleetly to promote mature beer. A safe strain Bacillus subtilis WB600 for high-yield production of ALDC was constructed with the ALDC gene saald from Staphylococcus aureus L3-15. SDS-PAGE analysis revealed that S. aureus α-acetolactate decarboxylase (SaALDC) was successfully expressed in recombinant B. siutilis strain. The enzyme SaALDC was purified using Ni-affinity chromatography and showed a maximum activity at 45 °C and pH 6.0. The values of K m and V max were 17.7 μM and 2.06 mM min(-1), respectively. Due to the unstable property of SaALDC at low pH conditions that needed in brewing process, site-directed mutagenesis was proposed for improving the acidic stability of SaALDC. Homology comparative modeling analysis showed that the mutation (K52D) gave rise to the negative-electrostatic potential on the surface of protein while the numbers of hydrogen bonds between the mutation site (N43D) and the around residues increased. Taken together the effect of mutation N43D-K52D, recombinant SaALDCN43D-K52D showed dramatically improved acidic stability with prolonged half-life of 3.5 h (compared to the WT of 1.5 h) at pH 4.0. In a 5-L fermenter, the recombinant B. subtilis strain that could over-express SaALDCN43D-K52D exhibited a high yield of 135.8 U mL(-1) of SaALDC activity, about 320 times higher comparing to 0.42 U mL(-1) of S. aureus L3-15. This work proposed a  strategy for improving the acidic stability of SaALDC in the  B. subtilis host.

Metabolic engineering of Candida glabrata for diacetyl production

In this study, Candida glabrata, an efficient pyruvate-producing strain, was metabolically engineered for the production of the food ingredient diacetyl. A diacetyl biosynthetic pathway was reconstructed based on genetic modifications and medium optimization. The former included (i) channeling carbon flux into the diacetyl biosynthetic pathway by amplification of acetolactate synthase, (ii) elimination of the branched pathway of α-acetolactate by deleting the ILV5 gene, and (iii) restriction of diacetyl degradation by deleting the BDH gene. The resultant strain showed an almost 1∶1 co-production of α-acetolactate and diacetyl (0.95 g L(-1)). Furthermore, addition of Fe3+ to the medium enhanced the conversion of α-acetolactate to diacetyl and resulted in a two-fold increase in diacetyl production (2.1 g L(-1)). In addition, increased carbon flux was further channeled into diacetyl biosynthetic pathway and a titer of 4.7 g L(-1) of diacetyl was achieved by altering the vitamin level in the flask culture. Thus, this study illustrates that C. glabrata could be tailored as an attractive platform for enhanced biosynthesis of beneficial products from pyruvate by metabolic engineering strategies.

A food-grade site-directed mutagenesis system for Streptococcus thermophilus LMG 18311

AIMS

To develop a general method for site-directed mutagenesis in the dairy starter strain Streptococcus thermophilus LMG 18311 which does not depend on antibiotic-resistance genes or other selection markers for the identification of transformants.

METHODS AND RESULTS

In a previous study, we demonstrated that Strep. thermophilus LMG 18311 can be made competent for natural genetic transformation by overexpression of the alternative sigma factor ComX. In the present study, we wanted to investigate whether the natural transformation mechanism of Strep. thermophilus LMG 18311 is efficient enough to make it feasible to perform site-directed mutagenesis in this strain without the use of a selection marker. Competent bacteria were mixed with a DNA fragment engineered to contain a nonsense and a frameshift mutation in the middle of the target gene (lacZ) and subsequently seeded on agar plates. By performing colony-lift hybridization using a digoxigenin-labelled oligonucleotide probe, we succeeded in identifying transformants containing the sought after mutation.

CONCLUSIONS

By exploiting the natural transformability of Strep. thermophilus LMG 18311 and standard molecular methods, we have demonstrated that the genome of this bacterium can be altered at preselected sites without introduction of any foreign DNA.

SIGNIFICANCE AND IMPACT OF THE STUDY

A food-grade site-directed mutagenesis system has been developed for Strep. thermophilus LMG 18311 that can be used by the dairy industry to construct starter strains with novel and/or improved properties.