BAP2, A GENE ENCODING A PERMEASE FOR BRANCHED-CHAIN AMINO-ACIDS IN SACCHAROMYCES-CEREVISIAE

To select the gene coding for an isoleucine permease, an isoleucine de pendent strain (ival chal) was transformed with a yeast genomic multic opy library, and colonies growing at a low isoleucine concentration we re selected. Partial sequencing of the responsible plasmid insert reve aled the presence of a previously sequenced 609 codon open reading fra me of chromosome Il with homology to known permeases. Deletion, extra dosage and C-terminal truncation of this gene were constructed in a st rain lacking the general amino acid permease, and amino acid uptake wa s measured during growth in synthetic complete medium. The following o bservations prompted us to name the gene BAP2 (branched-chain amino ac id permease). Deletion of BAP2 reduced uptake of leucine, isoleucine a nd valine by 25-50%, while the uptake of 8 other L-alpha-amino acids w as unaltered or slightly increased. Introduction of BAP2 on a centrome re-based vector, leading to a gene dosage of two or slightly more, cau sed a 50% increase in leucine uptake and a smaller increase for isoleu cine and valine. However, when the 29 C-terminal codons of the plasmid -borne copy of BAP2 were substituted, the cells more than doubled the uptake of leucine, isoleucine and valine, while no or little increase in uptake was observed for the other 8 amino acids.