Construction of poly-met DNA yeast hybrids for increased methionine content: technofunctional properties of the hybrid yeasts.

Methionine is a limiting essential amino acid in human nutrition, to overcome the possible overdosage and improve bioavailability methionine supply should be in protein bound form. So insertion a poly-met encoding DNA sequence results a more efficient solution in increasing methionine content of yeast. Poly-met DNA yeast hybrids were constructed of Saccharomyces cerevisiae CB89, an auxotrophic mutant strain. Synthetic DNA sequence encoding methionine polypeptide was inserted into the polylinker region of pVT-U 100 vector with 2 mu plasmid replicon. After transformation of E. coli HB101 cells the efficiency of the ligation and transformation was checked by digesting the minipreps. S. cerevisiae CB89 was transformed with vector-poly-met insert and with the plasmid vector only as well. Hybrid yeasts were selected on uracilless medium. PVT-U 100 can be used as vector for the expression of DNA sequence in S. cerevisiae. The vector harbours the promoter of ADC1 gene immediately downstream from the promoter lies a polylinker sequence comprising unique restriction enzyme sites for BamHI, HindIII, PvuII, SacI, Xhol. The polylinker sequence is followed by the transcriptional stop site and polyadenylation signal of ADC1 gene. Plasmid pVT-U 100 has selection markers for S. cerevisiae (URA3) and for E. coli (amp, per F). Results show that the poly-met DNA hybrids methionine content is influenced by the length of the insert. Fusion hybrids containing 600 bp oligo insert showed the best values. Distribution of methionine content in the protein subfractions of polymet DNA hybrid and parent strain CB89 was determined in dependence of glucose concentration and aeration intensity. The increase in synthesized methionine appeared in fractions 1 + 2 and residue. Technofunctional properties of parent strains and hybrids were compared for whole cells and cell wall (residue). Results demonstrate that enrichment in methionine in the cell wall fraction resulted improvement of emulsifying ability. Bioavailability of methionine content was better in DNA hybrid yeast than in parent strain and was the best when propagated in whey medium.