The 5' upstream region of the gene encoding isocitrate lyase of Candida tropicalis (UPR-ICL) is functional as a promoter in Saccharomyces cerevisiae, and it is regulated by carbon source; the expression of the gene is repressed when cells are grown on glucose, while it increases to a higher level in acetate-grown cells. Therefore, we have investigated regions in UPR-ICL responsible for gene expression in glucose-grown and acetate-grown cells. In glucose-grown cells, a deletion of the region between -801 and -569 (region G1) significantly decreased gene expression compared with that observed with the complete UPR-ICL. The region from -421 to -379 (region G2) also repressed gene expression in glucose-grown cells. In acetate-grown cells, two regions were found to strongly enhance gene expression, one between -728 and -569 (region A1) and the other between -370 and -356 (region A2). Whereas region A2 contained a sequence motif similar to the carbon-source-responsive element (CSRE), which mediates regulation by carbon source of S. cerevisiae ICL1, region A1 did not show similarity to any reported cis-acting elements. Deletion mutants of UPR-ICL containing only one of these regions showed that each region could independently activate gene expression to a similar level when the cells were grown on acetate. The influences of null mutations in the MIG1, SNF1 and CAT8 genes on regulation of UPR-ICL-mediated gene expression were examined. Expression of the ICL gene with full-length UPR-ICL increased about tenfold in mig1 cells grown on glucose, while little difference was observed in acetate-grown cells. The effects of snf1 and cat8 mutations were different between region-Al-mediated and region-A2-mediated gene expression in acetate-grown cells. Region-A2-mediated expression decreased 95% and 86% in snf1 and cat8 cells, respectively, while region-A1-mediated expression decreased 72% in snf1 cells and was not affected by the cat8 mutation. This finding indicates that region-A1-mediated gene expression is regulated by a pathway independent of CAT8, which is necessary for derepression of CSRE-mediated gene expression in S. cerevisiae.