The NGFI-B Gene, a Transcriptionally Inducible Member of the Steroid Receptor Gene Superfamily: Genomic Structure and Expression in Rat Brain after Seizure Induction

Abstract

The NGFI-B cDNA was previously isolated by virtue of its induction by nerve growth factor (NGF) in PC12 cells. It encodes a 61-kilodalton protein that has two regions of extensive homology with members of the steroid/thyroid hormone receptor gene family. The rat NGFI-B gene is approximately 7.6 kilobases long and is interrupted by six introns. Although the exon-intron structure of the gene is similar to those of several other members of the steroid/thyroid hormone receptor gene family, there is a novel splice site within the DNA-binding domain which suggests that NGFI-B constitutes yet another evolutionary digression from a postulated common ancestral receptor gene. Primer extension and SI nuclease protection assays were used to determine the transcription initiation site, which displayed the heterogeneity typical of genes that lack a TATA box. Sequence analysis of the 5′ flanking region revealed several GC boxes but no identifiable TATA box. Four potential API binding sites were identified at nucleotides −49, −78, −222, and −242. Neither the serum response element nor the CArG box element, two sequences found in other growth factor-inducible genes, was detected in this region of the growth factor-inducible NGFI-B gene. Nevertheless, results of nuclear runoff experiments demonstrated that the NGFI-B gene was transcriptionally activated by nerve growth factor in PC12 cells. In vivo, a rapid, dramatic increase in NGFI-B mRNA was observed in the cerebral cortex, midbrain, and cerebellum of animals that experienced a convulsant-induced seizure.