Expression of histone H5, like that of other erythrocyte specific proteins, declines during the latter stages of erythroid maturation because of a decrease in the rate of gene transcription. Here, we report the isolation of cIBR (chicken initiation binding repressor), a 75 kDa DNA binding glycoprotein from mature chicken erythrocytes that recognizes sequences spanning the transcription start sites of the H5 gene. cIBR was found to repress transcription from the H5 promoter in vitro and this effect could be relieved by mutations that lowered the affinity of the factor for its cognate sequence. cIBR inhibited transcription by interfering with assembly of the initiation complex, but it did not affect transcription from pre‐assembled complexes. Consistent with this, binding of bacterially expressed human TFIID to the TATA element prevented subsequent binding of cIBR, although the opposite was not true. This, and the fact that cIBR had no effect when bound in a location upstream from the promoter, suggests that binding of cIBR to the start site region causes repression by direct interference with general transcription factors other than TFIID, possibly TFIIB. cIBR was found in mature and relatively late erythrocytes but not in early erythroid cells which actively transcribe the H5 gene; the transcriptionally active cells contain instead cIBF (chicken initiation binding factor). Purified cIBF is a non‐glycosylated 68–70 kDa DNA binding protein(s) which also recognizes the region of transcription initiation of the H5 gene.

Initiation binding receptor (IBR) is a chicken erythrocyte factor (apparent molecular mass, 70 to 73 kDa) that binds to the sequences spanning the transcription initiation site of the histone h5 gene, repressing its transcription. A variety of other cells, including transformed erythroid precursors, do not have IBR but a factor referred to as IBF (68 to 70 kDa) that recognizes the same IBR sites. We have cloned the IBR cDNA and studied the relationship of IBR and IBF. IBR is a 503-amino-acid-long acidic protein which is 99.0% identical to the recently reported human NRF-1/␣-Pal factor and highly related to the invertebrate transcription factors P3A2 and erected wing gene product (EWG). We present evidence that IBR and IBF are most likely identical proteins, differing in their degree of glycosylation. We have analyzed several molecular aspects of IBR/F and shown that the factor associates as stable homodimers and that the dimer is the relevant DNA-binding species. The evolutionarily conserved N-terminal half of IBR/F harbors the DNA-binding/dimerization domain (outer limits, 127 to 283), one or several casein kinase II sites (37 to 67), and a bipartite nuclear localization signal (89 to 106) which appears to be necessary for nuclear targeting. Binding site selection revealed that the alternating RCGCRYGCGY consensus constitutes high-affinity IBR/F binding sites and that the direct-repeat palindrome TGCGCATGCGCA is the optimal site. A survey of genes potentially regulated by this family of factors primarily revealed genes involved in growth-related metabolism.Histone H5 is a specific linker histone of the erythroid lineage which is expressed during the maturation of embryonic and adult avian erythrocytes (reference 2 and references therein). The chromatin levels of H5 increase progressively following activation of the h5 gene, so that at the end of the maturation process, the stoichiometry of linker histones (H5 plus H1) per core histone octamer is more than 1 (5). The effects of ectopic expression of H5 in nonerythroid cells, to levels comparable to those found in erythrocytes, argues for a direct role of H5 in the condensation and inactivity of the erythrocyte nucleus (81,82).Although the h5 gene is already set in an active chromatin conformation in early erythroid precursors (69), its transcription is not activated until the preerythroblast-to-erythroblast transition (2, 72), coincident with the decline in the proliferation potential of the cells. The mechanisms that bring about transcriptional activation of the h5 gene are not fully elucidated but appear to reflect alterations in the balance between positive and negative effectors. Transcription of the h5 gene is regulated by a complex assortment of ubiquitous and blood cell-specific factors acting at the promoter and three differentiation-specific enhancers located upstream and downstream of the gene (72,73). Although the majority of these factors contribute to transcriptional activity, one of them, IBR, was found to repress h5 transcription in vitro (25). IB...