Genomic organization of IFI16, an interferon-inducible gene whose expression is associated with human myeloid cell differentiation: correlation of predicted protein domains with exon organization
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- Trapani, J.A., Dawson, M., Apostolidis, V.A. et al. Immunogenetics (1994) 40: 415. doi:10.1007/BF00177824
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The human IFI16 gene is a member of an interferon-inducible family of mouse and human genes closely linked on syntenic regions of chromosome 1. Expression of these genes is largely restricted to hemopoietic cells, and is associated with the differentiation of cells of the myeloid lineages. As a prelude to defining the mechanisms governing IFI16 expression, we have deduced its genomic organization using a combination of genomic cloning and polymerase chain reaction amplification of genomic DNA. IFI16 consists of ten exons and nine intervening introns spanning at least 28 kilobases (kb) of DNA. The reiterated domain structure of IFI16 protein is closely reflected in its intron/exon boundaries, and may represent the evolutionary fusion of several independent functional domains. Thus, exon 1 consists of 5' untranslated (UT) sequences and contains sequence motifs that may confer interferon-inducibility, and exon 2 encodes the lysine-rich amino-terminal (“K”) region, which possesses DNA-binding activity. Exon 3 codes for a domain which is poorly conserved between family members, except for a strongly retained basic motif likely to provide nuclear localization. The first of two 200 amino acid repeat domains that are the hallmark of this family (domain A) is represented jointly on exons 4 and 5, which are reiterated as exons 8 and 9, respectively, to encode the second 200 amino acid domain (B). Two intervening serine-threonine-rich domains (C and C'), unique to IFI16, are each encoded by single exons of identical length (exons 5 and 6). These domains are predicted to encode semi-rigid “spacer” domains between the 200 amino acid repeats. The reiterated nature of exons 4 to 6 and the insertion of introns into a single reading frame strongly suggest that IFI16 and related genes arose by a series of exon duplications, some of which antedated speciation into mouse and humans. Several alternative mRNA cap sites downstream of a TATA consensus sequence were defined, using primer extension analysis of mRNA. Sequencing of ~1.7 kb of DNA upstream of this region revealed no recognizable consensus elements for induction by interferon-α (interferon-α/ß-stimulated response elements), but two motifs resembling interferon-γ activation sites were located. IFNs α and γ both induce IFI16 mRNA expression in myeloid cells. Interferon-α inducibility of IFI16 may be regulated by an interferon-α/ß-stimulated response consensus element in the 5' UT exon, as a similar motif is conserved in the corresponding position in the related myeloid cell nuclear differentiation antigen gene. An interferon-γ-activation site consensus was also located in this region of IFI16.