Members of the Y-box (YB) family of transcription factors are expressed in a wide range of cell types and are implicated in the regulation of a rapidly increasing number of genes. Although the biological activities of YB proteins appear to be varied, distinct patterns, relating to the timing of their expression and the identity of their target genes, are beginning to emerge. A recent report by Ito et al. (') focusses attention on cell proliferation and adds support to earlier ~uggestions(*~~) that a primary function of YB proteins is to help activate growth-associated genes.
YB proteinc
The Y-box (YB) proteins comprise a family of gene regulators, each containing a nucleic acid-binding domain which is highly conserved from bacteria to h~mans(~1~). The archaetypal form is represented by the bacteria1 coldshock proteins, which consist of a five-stranded P-barre1 structure: the three N-terminal B-strands presenting a face with exposed aromatic and basic side-chains to interact with the nucleic acid (see ref. 6). Eukaryotic Y 6 proteins consist of this conserved cold-shock domain (CSD) plus a series of alternating basic and acidic domains, which may specify further features of nucleic acid binding and proteinprotein interaction. To date, most studies have focussed on the binding of Y 6 proteins to the Y-box DNA sequence CTGATTGGYYUU, which contains a reverse CCAATbox. The Y-box is present in the promoter region of a variety of eukaryotic genes and binding of YB proteins can result in either activation or repression of transcription. Tissue-specific expression of YB proteins indicates that they are involved in the regulation of distinct sets of genes. One set, already described, includes various vertebrate germcell-specific gened4): a second set, now becoming apparent, are genes involved in cell proliferation. Recent studies have highlighted cell types that are stimulated to express high levels of Y6 protein, which in turn may activate growth-associated genes and suppress tissue-specific genes.