BRAIN AND TESTIS SELECTIVE EXPRESSION OF THE GLUTATHIONE-S-TRANSFERASE YB-3 SUBUNIT IS GOVERNED BY TANDEM DIRECT REPEAT OCTAMER MOTIFS IN THE 5'-FLANKING REGION OF ITS GENE

To gain insight into mechanisms of cell type-specific transcription of class mu-glutathione S-transferase genes, the gene encoding the Yb-3 subunit was cloned. Yb-3 subunits are selectively expressed at high levels in rat brain and testis but not in liver or kidney. The Yb-3 subunit gene spans over 6 kb and consists of 8 exons and 7 introns and a sequence consisting of tandem direct repeat consensus octamer DNA binding motifs separated by a 6 base pair (bp) spacer was identified in its 5'-flanking region. Gel shift assays with a 40 bp segment of DNA containing the two consensus octamer sequences, revealed the presence of specific binding proteins in nuclear extracts of rat brain, testis and C6 glioma cells. DNA binding activity was greatly reduced in liver, kidney and HTC cells. Reporter vectors carrying segments of the 5'-flanking region of the Yb-3 subunit gene fused to a luciferase gene were introduced into C6 glioma cells which express high levels of Yb-3 subunits, and into HTC cells which do not. The plasmids consisting of the Yb-3 gene promoter up to, but not including, the octamer motifs did not support luciferase transcription in the C6 glioma cells, but larger fragments that included the octamer repeat sequences, effectively directed transcription in the C6 glioma cells. With mutated octameric sequences transcriptional activity was greatly reduced, and none of the same Yb-3 constructs directed substantial luciferase transcription in the HTC cells. The results show that octamer motifs in the 5'-flanking region of the Yb-3 subunit gene are functional and are the principal cis-acting elements that account for its discrete cell type-selective expression. This gene is one of the few known targets for octamer DNA binding transcription factors in brain.