C-MYB INTRON-I PROTEIN-BINDING AND ASSOCIATION WITH TRANSCRIPTIONAL ACTIVITY IN LEUKEMIC-CELLS

Specific binding of nuclear proteins to the region of transcriptional attenuation has been shown to modulate the expression of c-myb, a nucl ear proto-oncogene preferentially expressed in lympho-hematopoietic ce lls. Here, it plays an important role in processes of differentiation and proliferation. The mechanism that regulates c-myb expression is no t yet fully understood. The block of transcriptional elongation which has been mapped to a 1 kb region within murine intron 1 may represent one regulatory pathway. The DNA sequences containing the transcription al pause site are well conserved between murine and human species, thu s implying similar transcription-control strategies. We compared the b inding potential of nuclear extracts (from human fibroblasts and MOLT4 as well as murine NIH3T3- and 70Z/3B- cell lines) to oligonucleotide sequences previously shown to be target binding sites in the murine sy stem. One complex containing a 70 D protein was found to be associated specifically with transcriptionally active leukemia cells. We perform ed transient expression studies with a CAT reporter construct containi ng this putative enhancer sequence and yielded significant CAT activit y. We identified further a putative 20 kD repressor protein in transcr iptionally silent cells and demonstrated that c-Jun is part of an ubiq uitously present complex. Our results confirm the participation of int ron 1 in transcriptional regulation of the c-myb gene (in mouse and hu man) and implicate multiple and complex regulatory mechanisms of activ ation during myelomonocytic differentiation and leukemic cell growth c ontrol. Copyright (C) 1996 Elsevier Science Ltd.