The organization and transcription of the galactose gene cluster of Saccharomyces

Abstract The transcriptional activity of a kb † region of Saccharomyces DNA centered on three galactose-inducible sequences has been examined. An in vivo-generated gal7−, gal10−, gal1− mutation, gal▽ (isolated by D. C. Hawthorne), is a 7 kb deletion that overlaps the map positions of all three inducible RNAs. These three RNAs correspond to the transcripts of the GAL7-GAL10-GAL1 gene cluster located on chromosome II. Gene assignments have been inferred by comparison of the published protein sizes and the experimentally determined messenger RNA lengths. The physically assigned gene order, GAL7-GAL10-GAL1, agrees with the genetically established order. The GAL7 and GAL10 genes (which encode galactose-1-phosphate uridylyl transferase and uridine diphosphogalactose-4-epimerase, respectively) are transcribed from the same DNA strand. The GAL1 gene (which encodes galactokinase) is transcribed from the complementary DNA strand such that its 5′ end is 0.65 kb from the 5′ end of the GAL10 gene. These three genes are normally transcribed into at least six discrete RNA species in the strains examined. Two additional GAL1 transcripts have been detected at the non-permissive temperature in the yeast rna1-1 “nuclear transport” mutant. None of these eight transcripts contains detectable intervening sequences. Transcription of the GAL1 gene results in the presence of five detectable RNA molecules (1.65, 1.75, 4.5, 5.5 and 6.5 to 7 kb), the two longest having only been detected in the rna1-1 mutant at the restrictive temperature. These five transcripts differ only at their 3′ ends. The 5′ ends of these RNAs are indistinguishable by S1 nuclease mapping procedures. The two smallest RNAs are only slightly larger than required to encode galactokinase. GAL7 and GAL10 gene sequences are found in 1.25 and 2.25 kb RNA molecules. respectively, and are separated by about 0.65 kb. These separate GAL7 and GAL10 RNAs arc only approximately 200 bases longer than required to encode their respective proteins. These two genes are also transcribed together into a large RNA molecule (4.15 kb), 0.65 kb longer than the sum of the lengths of the separate GAL7 and GAL10 messenger RNAs. The 5′ end of this molecule cannot be distinguished from the 5′ end of the GAL10 mRNA. Its 3′ end is very near or identical to the 3′ end of the GAL7 mRNA. The yeast rna1-1 mutant, deficient in nuclear transport (and/or RNA processing), accumulates the three largest GAL1 transcripts at the non-permissive temperature. Under these conditions, the rna1-1 mutant also accumulates the large RNA containing both GAL7 and GAL10 gene sequences as well as the 1.25 kb GAL7 RNA. These data suggest the presence of a promoter for the transcription of the GAL1 gene, an adjacent promoter for the transcription of GAL10 and/or both GAL10 and GAL7, and a third promoter for the GAL7 gene. The 28 kb of cloned DNA sequences extend beyond the gene cluster and contain at least four transcribed regions that are not likely to be related to the GAL gene cluster. All of the DNA in the cluster region itself is unique, although repeated DNA sequences are located at the extreme left edge of the cloned region.