The lymphoid transcription factor LyF-1 is encoded by specific, alternatively spliced mRNAs derived from the Ikaros gene.

The lymphocyte-specific DNA-binding protein LyF-1 interacts with a critical control element in the terminal deoxynucleotidyltransferase (TdT) promoter as well as with the promoters for other genes expressed during early stages of B- and T-cell development. We have purified LyF-1 and have obtained a partial amino acid sequence from proteolytic peptides. The amino acid sequence suggests that LyF-1 is a zinc finger protein encoded by the Ikaros gene, which previously was implicated in T-cell development. Recombinant Ikaros expressed in Escherichia coli bound to the TdT promoter, and antisera directed against the recombinant protein specifically blocked the DNA-binding activity of LyF-1 in crude extracts. Further analysis revealed that at least six distinct mRNAs are derived from the Ikaros/LyF-1 gene by alternative splicing. Only two of the isoforms possess the N-terminal zinc finger domain that is necessary and sufficient for TdT promoter binding. Although both of these isoforms bound to similar sequences in the TdT, lambda 5, VpreB, and lck promoters, one isoform contains an additional zinc finger that resulted in altered recognition of some binding sites. At least four of the Ikaros/LyF-1 isoforms were detectable in extracts from B- and T-cell lines, with the relative amounts of the isoforms varying considerably. These data reveal that the LyF-1 protein is encoded by specific mRNAs derived from the alternatively-spliced Ikaros gene, suggesting that this gene may be important for the early stages of both B- and T-lymphocyte development.