Identification of the plasma membrane H+-biotin symporter of Saccharomycescerevisiae by rescue of a fatty acid-auxotrophic mutant

Bakers' yeast is auxotrophic for biotin (vitamin H) and depends on the effi cient uptake of this compound from the environment. A mutant strain with st rongly reduced biotin uptake and with reduced levels of protein biotinylati on was identified. The strain was auxotrophic for long-chain fatty acids, a nd this auxotrophy could be suppressed with high levels of biotin in the me dium. After transformation of this mutant with a yeast genomic library, the unassigned open reading frame YGR065C was identified to complement this mu tation. This gene codes for a protein with 593 amino acids and 12 putative transmembrane helices, Northern blot analysis revealed that, in wild-type c ells, the corresponding mRNA levels were increased at low biotin concentrat ions. Likewise, cellular biotin uptake was increased with decreasing biotin availability. Expression of YGR065C under the control of the constitutive ADH1 promoter resulted in very high biotin transport rates across the plasm a membrane that were no longer regulated by the biotin concentration in the growth medium. We conclude that YGR065C encodes the first biotin transport er identified for a non-mammalian organism and designate this gene VHT1 for vitamin H transporter 1.