ABSTRACT
Previous studies have revealed the presence of at least two histidine uptake systems in S. cerevisiae; one with high affinity and the other with low affinity for histidine. The HIP1 gene is known to encode the high affinity permease. The purpose of this study was to identify the gene that encodes the low affinity permease. A mutant strain of S. cerevisiae that is both a histidine auxotroph and a hip1 deletion mutant is unable to grow on low histidine media. This strain was transformed with a yeast cDNA library constructed in a yeast expression vector. Transformants with increased histidine transport were selected by their ability to grow on a low histidine media. Sequencing of the inserts revealed the presence of the HIP1 gene and also the presence of the TAT1 gene. Estimated Km and Vmax values for histidine transport by each system were determined. In a hip1 tat1 double mutant, the level of histidine required for growth increased eight-fold in comparison to the hip1 single mutant. Our results suggest that the TAT1-encoded protein, previously characterized as the high-affinity tyrosine permease, also acts as the low affinity histidine permease.
