CLONING AND CHARACTERIZATION OF CHICKEN YB-1 - REGULATION OF EXPRESSION IN THE LIVER

A cDNA expression library constructed from day 9 embryonic liver was screened with a previously identified protein binding site in the flanking region of the liver-specific, estrogen-dependent avian apoVLDLII gene. Two of the clones isolated were shown to encode the chicken homolog of the Y-box binding protein, YB-1 (dbpb), which we have designated chkYB-1. This protein was originally identified in avian extracts by virtue of its ability to bind to two reverse CCAAT motifs in the Rous sarcoma virus enhancer. Since its identification, additional nucleic acid binding properties have been ascribed to its homologs, or closely related proteins, in other species. We have determined the sequence of chkYB-1, investigated its ability to bind to sites known to be involved in tissue-specific expression in the liver, and examined factors influencing its hepatic expression. These studies have demonstrated that the level of chkYB-1 mRNA in the liver decreases steadily throughout embryogenesis and for several weeks posthatching until adult levels are attained. We present several lines of evidence that YB-1 expression in the liver is positively associated with DNA synthesis or cell proliferation. Its binding characteristics indicate that the protein can interact specifically with a number of binding sites for liver-enriched or specific factors. In addition, although it is not particularly asymmetric in terms of base composition, we find a marked preference in binding to the pyrimidine-rich strand of these sites regardless of the presence or polarity of an intact CCAAT box. The increased levels of expression of YB-1 during proliferation combined with its binding characteristics suggest that it may be involved in the reduced expression of liver-specific genes observed at early stages of development or during liver regeneration.