The GAL1 and GAL10 genes of Saccharomyces cerevisiae are divergently transcribed, with 606 base pairs of DNA separating their transcription initiation sites. These genes are tightly coregulated, with their expression induced approximately 1,000-fold in cells growing on galactose and repressed by growth on glucose. The DNA sequence between these genes and the precise sites of transcription initiation have been determined. A 108-base-pair guanine-plus-cytosine-rich region is located approximately in the middle of this region. Mutations in this region suggest that it is required for the expression of both genes. The DNA between GAL1 and GAL10 is sufficient for regulation of these genes, as shown by the fusion of this region to the yeast HIS3 gene, placing HIS3 under GAL control. The GAL1, GAL7, and GAL10 genes, which encode enzymes for galactose utilization, are clustered near the centromere of chromosome II. These genes are separately transcribed from individual promoters and are stringently coregulated. Regulation occurs at the transcriptional level. Genetic analyses identified two genes, GAL4 and GAL80, involved in regulation of GAL gene expression. GAL4 encodes a protein required for expression of GAL1, GAL7, and GAL10, while GAL80 encodes a protein that antagonizes the GAL4 protein. The current model for GAL gene induction proposes that the inducer (galactose or its metabolites) alters the GAL80 protein, preventing it from binding to the GAL4 protein, thus inducing GAL1, GAL7, and GAL10 expression. The GAL1 and GAL10 genes are adjacent and transcribed in opposite directions, with approximately 600 base pairs of DNA separating their transcription initiation sites. Results suggest that the 600-base-pair region of DNA carries sequences responsible for regulation of GAL1 and GAL10. The DNA sequence of the region between GAL1 and GAL10 and the location of sites of transcription initiation for these two genes are presented. Mutations constructed in vitro or selected in vivo that alter the region between GAL1 and GAL10 and affect expression of these genes are also described. The study also includes methods for DNA sequencing, determination of transcription initiation sites, preparation of probes, construction of deletions in vitro, and transplacement of deleted GAL sequences into the yeast genome. The results show that sequences in the centrally located G+C-rich region are essential for both GAL1 and GAL10 expression. Mutations selected in vivo that prevent GAL1 expression are all DNA rearrangements, suggesting few point mutations are sufficient for the selection. The study also discusses the utility of GAL-HIS3 fusions for studying GAL gene regulation and the potential of the plasmid carrying the GAL1-10 control region for high-level expression of a wide variety of genes in yeThe GAL1 and GAL10 genes of Saccharomyces cerevisiae are divergently transcribed, with 606 base pairs of DNA separating their transcription initiation sites. These genes are tightly coregulated, with their expression induced approximately 1,000-fold in cells growing on galactose and repressed by growth on glucose. The DNA sequence between these genes and the precise sites of transcription initiation have been determined. A 108-base-pair guanine-plus-cytosine-rich region is located approximately in the middle of this region. Mutations in this region suggest that it is required for the expression of both genes. The DNA between GAL1 and GAL10 is sufficient for regulation of these genes, as shown by the fusion of this region to the yeast HIS3 gene, placing HIS3 under GAL control. The GAL1, GAL7, and GAL10 genes, which encode enzymes for galactose utilization, are clustered near the centromere of chromosome II. These genes are separately transcribed from individual promoters and are stringently coregulated. Regulation occurs at the transcriptional level. Genetic analyses identified two genes, GAL4 and GAL80, involved in regulation of GAL gene expression. GAL4 encodes a protein required for expression of GAL1, GAL7, and GAL10, while GAL80 encodes a protein that antagonizes the GAL4 protein. The current model for GAL gene induction proposes that the inducer (galactose or its metabolites) alters the GAL80 protein, preventing it from binding to the GAL4 protein, thus inducing GAL1, GAL7, and GAL10 expression. The GAL1 and GAL10 genes are adjacent and transcribed in opposite directions, with approximately 600 base pairs of DNA separating their transcription initiation sites. Results suggest that the 600-base-pair region of DNA carries sequences responsible for regulation of GAL1 and GAL10. The DNA sequence of the region between GAL1 and GAL10 and the location of sites of transcription initiation for these two genes are presented. Mutations constructed in vitro or selected in vivo that alter the region between GAL1 and GAL10 and affect expression of these genes are also described. The study also includes methods for DNA sequencing, determination of transcription initiation sites, preparation of probes, construction of deletions in vitro, and transplacement of deleted GAL sequences into the yeast genome. The results show that sequences in the centrally located G+C-rich region are essential for both GAL1 and GAL10 expression. Mutations selected in vivo that prevent GAL1 expression are all DNA rearrangements, suggesting few point mutations are sufficient for the selection. The study also discusses the utility of GAL-HIS3 fusions for studying GAL gene regulation and the potential of the plasmid carrying the GAL1-10 control region for high-level expression of a wide variety of genes in ye