3 RESULTS
3.1 Auxotrophs of S. cerevisiae decreased both cell growth and FFA production
Eight auxotrophs that are deficient in synthesis of some specific amino acids and nucleotides, including ura3 Δ, his3 Δ,met15 Δ, leu2 Δ, trp1 Δ, ade2 Δ, arg4 Δ, and lys2 Δ, were constructed in an FFA producing strain YJZ06[19]. These strains grew normally in minimal plates supplementing the corresponding nutrition (Figure S1). While cultivating in minimal media containing commonly used concentrations of 20 mg/L of uracil and histidine, 60 mg/L of leucine[14], 20 mg/L of methionine and arginine[21], 40 mg/L of tryptophan[22], 30 mg/L of lysine [23]and 50 mg/L of adenine [24], a serious decreased growth and FFA titers were observed. In particular, ura3 Δ,met15 Δ, leu2 Δ, arg4 Δ, and lys2 Δ strains had 13.5-35.0% of maximum biomass and 23.7-44.2% of FFA in compared with the prototrophic strain (Figure S2). In this case, we suspected that the supplemented amino acids and nucleotides were not enough to support cell growth and FFA production. We increased the concentrations of amino acids and nucleotides to 100 mg/L to cultivate the strains (Figure S3). Growth behaviors of strains like ura3 Δ, his3 Δ,arg4 Δ, and ade2 Δ had been greatly improved with a slight influence on FFA titers, but strains of leu2 Δ, met15 Δ,trp1 Δ, and lys2 Δ still grew poorly with an extremely low FFA titers, which demonstrated that effects of auxotrophic strains were distinguished, and different concentrations of amino acids and nucleotides were required. Therefore, we set 100 mg/L as the baseline to test the essential amounts of amino acids and nucleotides to restore cell growth and bio-productions.