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

A cDNA expression library constructed from day 9 embryonic liver was s creened with a previously identified protein binding site in the flank ing region of the liver-specific, estrogen-dependent avian apoVLDLII g ene. Two of the clones isolated were shown to encode the chicken homol og 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 a cid 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 involv ed in tissue-specific expression in the liver, and examined factors in fluencing 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 ar e attained. We present several lines of evidence that YB-1 expression in the liver is positively associated with DNA synthesis or cell proli feration. Its binding characteristics indicate that the protein can in teract specifically with a number of binding sites for liver-enriched or specific factors. In addition, although it is not particularly asym metric in terms of base composition, we find a marked preference in bi nding to the pyrimidine-rich strand of these sites regardless of the p resence or polarity of an intact CCAAT box. The increased levels of ex pression of YB-1 during proliferation combined with its binding charac teristics suggest that it may be involved in the reduced expression of liver-specific genes observed at early stages of development or durin g liver regeneration.