Genetic Characterization

Human PKR is encoded at position 21-22 on the short arm of chromosome 2 (chromosome 17 in mouse) (Tanaka and Samuel 1994, 1995; Kuhen et al. 1996a, 1996b; Xu and Williams 1998). The gene has 17 exons dispersed within a 50-kb genomic region. It has been shown that alternative splicing of exon 2 gives rise to three 5'-untranslated exons of different length (Kawakubo et al.

1999). Splice variants have been reported to generate kinases of variable activity from interferon (IFN)-treated U cells. An alternatively spliced form of PKR has also been reported in human T cell leukaemia Jurkat cells in which exon 7 is deleted, resulting in a truncated protein that retains the amino terminus but lacks the catalytic domain (Li and Koromilas 2001; Hii et al. 2004). This isoform acts as a dominant negative. PKR is constitutively expressed in all tissues at a basal level and is induced by type I IFNs. The promoter regions of the human and mouse pkr genes contain the same regulatory elements but differ in their precise arrangement (Kuhen and Samuel 1997). Basal expression is driven from a unique 15-nucleotide kinase conserved sequence element (KCS; GGGAAGGCGGAGTCC) that functions in concert with an interferon stimulatory response element (ISRE; GAAAACGAAACT) for inducible expression (Kuhen and Samuel 1999; Ward and Samuel 2002). The transcription factors Sp1 and Sp3 mediate basal expression, while IFN-inducible expression is Sp3 independent, but STAT1 and JAK-1 dependent (Das et al. 2006). An additional 40-base pair negative regulatory domain occurs approximately 400 bases upstream of the KCS element that works in concert with the KCS element to suppress transcription. The additional transcription factors Sp1, Sp3, STAT1, STAT2, IFN regulatory factor 9 (IRF9), p127DDB1 and p48DDB2 immuno-precipitate with the 5'-untranslated region of pkr (Ward and Samuel 2003; Das et al. 2004). Other transcription factors have been ascribed putative roles from sequence analysis of the gene promoter. These include Ets, Myb, MyoD, E2F, nuclear factor K B (NF-kB), and interleukin-6 activation factors (Tanaka and Samuel 1994). Together these regulatory elements distinguish a gene regulated by innate immune responses as well as cell growth and differentiation processes. There is also evidence that PKR expression is autoregulated in vivo at the level of translation (Thomis and Samuel 1992).

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