Bidirectional Gene Pair Architecture of the Regulatory Major Immediate Early Locus

It is widely accepted that the MIE locus of CMVs, which includes an essential enhancer region (Dorsch-Hasler et al. 1985; Ghazal et al. 2003; for a review, see Meier and Stinski 2006), is a key regulatory unit that kick-starts the viral transcriptional program in acute infection as well as in reactivation from latency (see the chapters by G. Maul; M.F. Stinski and D.T. Petrik, this volume). An open viral chromatin structure at the MIE locus appears to be a primary condition for reactivation to be initiated (Bain et al. 2006; see the chapter by M. Reeves and J. Sinclair, this volume). The MIE locus in mCMV is unique in that it is structurally organized as a bidirectional gene pair. Bidirectional gene pair architecture is defined as two neighboring genes arranged head-to-head on opposite strands of the DNA and regulated by a shared cis-acting regulatory unit. It has long been known that this organization applies to the mCMV MIE locus (Fig. 1) (for a review see Simon et al. 2006b). Interest in this structural feature of the mCMV MIE locus (Chatellard et al. 2007; Simon et al. 2007) results from the recent finding that bidirectional gene pairs are common in the human genome, often conserved among mouse orthologs and thought to provide a unique mechanism of regulation for a significant number of mammalian genes, in particular of genes involved in DNA repair (Adachi and Lieber 2002; Li et al. 2006; Trinklein et al. 2004). That such an architecture is used by mCMV just for the MIE locus, a locus of outstanding regulatory importance, is intriguing and underlines the close host-relatedness of this highly host-adapted virus. In this context, it is of interest to note that hCMV uses a bidirectional promoter element within oriLyt, another locus with a key function in acute infection and reactivation (Xu et al. 2004).

Recent data suggest that the mCMV MIE enhancer region is actually a tandem of two bona fide enhancers E1/3 and E2 (Chatellard et al. 2007) driving the transcription from genes m123/M122 (ie1/ie3) and m128 (ie2), respectively, in opposite directions. Alternative splicing of the IE1/IE3 precursor RNA leads to IE1 mRNA (coding exons 2, 3, and 4) and IE3 mRNA (coding exons 2, 3, and 5); splicing of IE2 precursor RNA leads to IE2 mRNA (coding exon 3).

Fig. 1 Bidirectional gene pair architecture of the mCMV MIE locus. Arrows indicate the direction of transcription. Numbered cylindrical boxes represent exons. C, complementary strand. Mand m indicate mCMV ORFs homologous to hCMV ORFs and mCMV private ORFs, respectively, according to the nomenclature used by Rawlinson et al. (1996). P promoter, E enhancer

Fig. 1 Bidirectional gene pair architecture of the mCMV MIE locus. Arrows indicate the direction of transcription. Numbered cylindrical boxes represent exons. C, complementary strand. Mand m indicate mCMV ORFs homologous to hCMV ORFs and mCMV private ORFs, respectively, according to the nomenclature used by Rawlinson et al. (1996). P promoter, E enhancer

The 76/89-kDa IE1 protein is involved in breaking epigenetic host cell defense by early disruption of nuclear domains (ND)10 (see the chapter by G. Maul, this volume); it co-transactivates the expression of viral early (E)-phase genes and autostimulates its own promoter. Interestingly, it also acts as a transactivator of cellular genes involved in dNTP biosynthesis, such as thymidylate synthase (Gribaudo et al. 2000) and ribonucleotide reductase (Lembo et al. 2000), a property thought to facilitate virus replication in resting cells (for reviews, see Simon et al. 2006b; Tang and Maul 2006). Clearly, efficient provision of dNTPs could possibly be a key parameter in virus reactivation from latently infected cells, which are most likely resting cells, and it will be intriguing to test this idea. The importance of the 88- to 90-kDa IE3 protein for virus reactivation is undoubted, as IE3 is the essential transactivator of viral E gene expression (Angulo et al. 2000). Thus, beyond MIE locus transcription initiation, differential splicing generating IE3 mRNA is a crucial second molecular checkpoint in the transition from mCMV latency to reactivation. Strikingly, no essential function could so far be identified for the 43-kDa IE2 protein (Cardin et al. 1995; Messerle et al. 1991). It remains an open question whether IE2 is really dispensable or whether we just failed to ask the correct questions, to design the proper experiments, and to look at the right place and time. In this context, it is of interest that recent work by Ishiwata et al. (2006) has localized IE2 protein in the brain of neonatally infected mice at a stage of prolonged infection selectively in neurons of the cortex and hippocampus, while the IE3 protein was preferentially expressed in glial cells only at an early stage of the infection. It is a challenge to identify the function of IE2 and to understand why its expression is paired with IE1 and IE3 in bidirectional gene organization, a gene architecture adopted from the mammalian host.

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