ISOLATION AND CHARACTERIZATION OF A CHICKEN HOMOLOG OF THE E2F-1 TRANSCRIPTION FACTOR

In higher eukaryotes, the E2F-1 transcription factor is an essential a nd limiting component of cell cycle progression in late G1. E2F-1 hete rodimerizes with members of the DP gene family and the resulting heter odimer regulates the expression of several protooncogenes and the gene tic machinery of DNA replication. Cell cycle regulation of E2F activit y is mediated through its association with the tumor suppressor Rb gen e product. To examine the evolutionary conservation of the E2F-1 prote in sequence and its developmental expression pattern we have isolated and sequenced the chick E2F-1 gene (chE2F-1) cDNA. The chicken protein is 34 amino acids (a.a) shorter than its human counterpart (403/437 a .a.) but has extremely web conserved bHLH and pRb binding domains, wit h respectively 94% and 83% identity. The position of the leucine zippe r is also strictly conserved thereby accounting for ability of E2F-1 t o form heterodimers with human and chicken DP-1. E2F-1 expression was analysed in synchronized cells as well as in embryonic or newborn chic k tissues and appears to be closely correlated to the cell proliferati on rate. In situ hybridization studies have shown very high expression levels in the neuroretina during the early stages of embryonic develo pment when active nenroblast division occurs. Tn contrast, a sharp dow n-regulation is observed when cells become postmitotic. Overexpression of the chE2F-1 protein leads to oncogenic transformation only when a truncated version of the transgene lacking the pRb binding domain is u sed; the full length protein either has no effect or may be deleteriou s for cell survival.