Adenovirus Type 2 Preferentially Stimulates Polymerase III Transcription of Alu Elements by Relieving Repression: a Potential Role for Chromatin

Abstract

The number of Alu transcripts that accumulate in HeLa and other human cells is normally very low; however, infection with adenovirus type 5 increases the expression of Alu elements dramatically, indicating that the potential for polymerase III (pol III)-dependent Alu transcription in vivo is far greater than generally observed (B. Panning and J. R. Smiley, Mol. Cell. Biol. 13:3231-3244, 1993). In this study, we employed nuclear run-on in combination with a novel RNase H-based assay to investigate transcription from uninfected and adenovirus type 2-infected nuclei, as well as genomic DNAs from uninfected and infected cells. When performed in the presence of excess uninfected nuclear extract, such assays revealed that (i) the vast majority of transcriptionally competent Alu elements in nuclei are masked from the pol III transcriptional machinery and (ii) the induction of Alu expression upon adenovirus infection can be largely accounted for by an increased availability of these elements to the pol III transcription machinery. We also investigated the role of H1 histone for silencing of Alu genes and, in comparison, mouse B2 repetitive elements. Depletion of H1 led to an Ϸ17-fold activation of B2 repetitive elements but did not change Alu transcription relative to that of constitutively expressed 5S rRNA genes. These results are consistent with the view that Alu repeats are efficiently sequestered by chromatin proteins, that such masking cannot be accounted for by nonspecific H1-dependent repression, and that adenovirus infection at least partially overrides the repressive mechanism(s).Within the human genome reside at least 500,000 copies of the Alu interspersed repetitive gene family (9, 45). These repeat elements possess bipartite RNA polymerase III (pol III) promoters and serve as active templates in vitro, yet despite their high copy number seldom give rise to high levels of pol III-transcribed RNAs in vivo. Important exceptions to this rule are provided by the recent findings that pol III-dependent Alu transcription can be strongly induced in HeLa cells following high-titer adenovirus or herpesvirus infection (36, 37). These findings confirm that Alu repeats have the potential to function as active templates in vivo under certain conditions but leave unanswered why such low Alu small-RNA levels accumulate under normal circumstances. One explanation which has been offered is that the vast majority of Alu repeat elements, by virtue of their insertion into the genome via retroposition, lack 5Ј-flanking sequences requisite for efficient capture of transcription factors (43). According to this idea, since Alu elements are unable to compete for pol III transcription factors (which are presumed to be present in limiting amounts within the cell), they are progressively packaged into chromatin structures with concomitant transcriptional silencing. The differential developmental regulation of Xenopus somatic and oocyte 5S genes has served as a paradigm in this regard. It appears that oocyte 5S genes are subject to chrom...

7SL RNA, but not the 54-kd signal recognition particle protein, is an abundant component of both infectious HIV-1 and minimal virus-like particles

“…The results demonstrated that the amount of 7SL RNA was twofold higher in HIV-infected than uninfected cells, while the amount of actin mRNA decreased twofold in HIV-infected cells, resulting in a fourfold increase in the intracellular ratio of 7SL to actin mRNA upon HIV infection (Table 1). The observation that HIV-1 infection leads to a fourfold increase in pol III-driven 7SL RNA expression compared with actin mRNA levels may be consistent with findings for several other viruses that are known to affect pol III-driven gene expression (Panning and Smiley 1995;Russanova et al 1995;Gottesfeld et al 1996;Piras et al 1996).…”

Retinoblastoma Protein Disrupts Interactions Required for RNA Polymerase III Transcription

“…Having confirmed the efficacy of TSA in murine fibroblasts, we examined how a chromosomal Pol III template responds to the drug. The B2 family was chosen for this analysis, since previous studies have shown that Pol III transcription of these middle repetitive genes is heavily repressed by histones in chromatin from murine fibroblasts (6,43). Consistent with these findings, treatment with TSA produced a strong induction of B2 transcripts (Fig.…”

DNA cleavage and Trp53 differentially affect SINE transcription

“…Available evidence suggests that murine SINE elements are particularly subject to this type of transcriptional control through interactions with chromatin proteins (Carey et al, 1986). Moreover, some treatments (adenoviral infection, heat shock) that have previously been shown to induce SINE transcription also modulate chromatin structure (Russanova et al, 1995;Kim et al, 2001). The activation of SINE transcription following treatment with DNA-damaging agents described in this study may be dependent on the same mechanism.…”

“…Alu elements tend to be located in areas of dense heterochromatin. Chromatin structure surrounding SINE elements may play a key role in sequestering these elements to transcriptionally silent regions of the genome (Ullu and Weiner, 1985;Carey et al, 1986;Russanova et al, 1995). Available evidence suggests that murine SINE elements are particularly subject to this type of transcriptional control through interactions with chromatin proteins (Carey et al, 1986).…”