REL NF-KB NUCLEAR-COMPLEXES THAT BIND KB SITES IN THE MURINE C-REL PROMOTER ARE REQUIRED FOR CONSTITUTIVE C-REL TRANSCRIPTION IN B-CELLS/

The c-rel protooncogene, a member of a transcription factor family tha t includes NF-kappaB, displays a complex pattern of gene expression. T o understand the basis of this expression, the regulatory region upstr eam of the murine c-rel transcription start sites has been cloned and characterized. Transcription of the murine c-rel gene initiates at mul tiple sites downstream of a GC-rich region conserved in the chicken c- rel promoter. This conserved region contains consensus transcription f actor binding sites for SP-1 and NF-kappaB (kB3 site) and is sufficien t for basal expression in Jurkat T-cells. In contrast, two additional NF-kappaB-like sites (kB1 and kB2) and an octamer consensus binding si te, all located upstream of the conserved region, are required for exp ression of promoter-reporter gene constructs in the B-cell line I29B. NF-kappaB sites kB1 and kB3 bind p50/65 and p50 homodimers, whereas kB 2 binds a distinct complex. The consensus octamer site, although only able to bind Oct1 and Oct2 with low affinity, appears to overlap with a binding site for a novel protein(s) expressed in I29B cells. Cotrans fection studies show that p75-c-rel and a carboxyl-terminal truncated c-rel protein that lacks the known trans-activating domain both up-reg ulate the c-rel promoter in 129B cells via a mechanism independent of the NF-kappaB motifs, whereas a mutant c-rel protein lacking the DNA b inding domain has no effect. Together, these findings suggest that, in this B-cell line, trans-activation of the c-rel promoter by rel prote ins is via an indirect mechanism.