Saccharomyces cerevisiae normally will not take up sterols from the environment 
under aerobic conditions. A specific mutant, upc2-1, of the predicted 
transcriptional activator UPC2 (YDR213w) has been recognized as a strain that 
allows a high level of aerobic sterol uptake. Another predicted transcriptional 
activator, the YLR228c gene product, is highly homologous to Upc2p. In fact, at 
the carboxy terminus 130 of the last 139 amino acids are similar between the two 
proteins. Since these proteins are very similar, the effect of mutations in the 
YLR228c open reading frame (ORF) was compared with like alterations in UPC2. 
First, the YLR228c ORF was insertionally inactivated and crossed with various 
UPC2 constructs. Deletion of YLR228c and UPC2 in combination resulted in 
nonviability, suggesting that the two proteins have some essential overlapping 
function. The upc2-1 point mutation responsible for aerobic sterol uptake was 
duplicated in the homologous carboxy region of the YLR228c ORF using 
site-directed mutagenesis. This mutation on a high-copy vector resulted in an 
increase in sterol uptake compared to an isogenic wild-type strain. The 
combination of both point mutations resulted in the greatest level of aerobic 
sterol uptake. When the YLR228c point mutation was expressed from a low-copy 
vector there was little if any effect on sterol uptake. Gas chromatographic 
analysis of the nonsaponifiable fractions of the various strains showed that the 
major sterol for all YLR228c and UPC2 combinations was ergosterol, the consensus 
yeast sterol.