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Glutamine synthetase (GS) expression increases dramatically during adipocyte differentiation of confluent 3T3-L1 cells. To identify differentiation-responsive cis-acting elements in the GS gene, several GSfusion genes were prepared and analyzed in stably transfected 3T3-L1 cells under conditions that trigger adipocyte differentiation. We find that the GS proximal 5'-flanking sequence lacks the regulatory elements required for differentiation-responsive expression. In contrast, a 2 kb intron 1 restriction fragment fused upstream of a heterologous promoter does drive reporter gene expression during hormone-triggered differentiation. The enhancer activity was localized to a 310 bp sequence near the middle of intron 1. Expression of fusion genes that include this 310 bp sequence does not temporally coincide with native gene expression. However, a composite gene that includes a far upstream GS sequence and the 2 kb intron 1 sequence yields a qualitatively different pattern of expression that closely resembles that of the native GS gene. The far upstream sequence alone exhibits no enhancer activity. Electrophoretic mobility shift analyses indicate that a 32 bp sequence within the 310 bp functional enhancer specifically binds differentiation-associated nuclear proteins. Although a C/EBP consensus sequence occurs in the 32 bp fragment, supershift analyses exclude C/EBP isoforms as the binding factor. In contrast, mutational analysis of the putative enhancer suggests that an HNF-3 isoform is involved. Thus our data indicate that elements in the distal 5'-flanking sequence and the first intron function cooperatively to regulate GS transcription and that HNF-3 may participate in that regulation.
Hadden et al. (Wed,) studied this question.