Polyoma virus early-late switch: regulation of late RNA accumulation by DNA replication.

Abstract

Early in infection of permissive mouse cells, messages from the early region of the polyoma virus genome accumulate preferentially over those from the late region. After initiation of DNA replication, the balance between early and late gene expression is reversed in favor of the late products. In previous work from our laboratory, we showed that viral early proteins do not activate the polyoma late promoter in the absence ofDNA replication. Here The molecular details underlying the early-late switch are poorly understood, although large TAg is known to play a pivotal role. Large TAg binds to its target DNA sequences within and adjacent to the replication origin (6-8) and is required for initiation and maintenance of polyoma virus replication. Previous experiments by others suggest that large TAg also transactivates the late promoter and represses transcriptional activity of early genes (2, 9-11). More recent data have suggested an additional involvement of middle TAg in regulating the early-late switch by interacting with host signal transduction pathways, which in turn modify transcription factors (10, 12). However, posttranscriptional regulation may play a more important role than large TAg in accumulation of late messages after DNA replication is initiated (3,(13)(14)(15)(16). Activation of polyoma virus late genes is replicationdependent because (i) polyoma mutants carrying temperature-sensitive large TAg mutations cannot proceed into the late phase at a nonpermissive temperature (2); (ii) cytosine arabinoside (araC), a DNA replication inhibitor, prevents the polyoma virus early-late switch (3,14); and (iii) polyoma virus late genes are generally not expressed in significantThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. levels in transformed nonpermissive cells, in which polyoma fails to replicate (15). Regulation of gene expression by DNA replication could be achieved via trans-or cis-acting mechanisms. Trans-acting factors may be modified, induced, or titrated as a result ofDNA replication. Alternatively, changes in chromatin structure induced by replication may exert a direct cis effect on late gene expression (17, 18). In simian virus 40 (SV40), late genes can be activated in trans by large TAg in the absence of DNA replication (19)(20)(21)(22). Recently, a trans effect involving depletion of an important transcriptional factor has been suggested for polyoma virus late gene activation (12). In contrast, previous reports presented evidence for a direct effect of the SV40 replication origin in activating the SV40 late promoter (23). Results from adenoviruses (24, 25) indicated cis-acting template effects on transcriptional control and posttranscriptional processing. A systematic study of a cis effect of DNA replication on polyoma late gene activation has not yet been reported.We show here that TAg expression in the absence of DNA replic...

Gene regulation by sense–antisense overlap of polyadenylation signals

“…We, and others, have reported that the early-to-late switch is characterized by a change in the efficiency of usage of the viral late polyadenylation signal. At early times, polyadenylation and transcription termination are efficient on the late strand; these transcripts are relatively unstable in the nucleus and accumulate to only low levels Carmichael 1988, 1990;Liu and Carmichael 1993;Liu et al 1994). At late times, however, polyadenylation and transcription termination become inefficient, leading to multigenomic primary transcripts (Acheson et al 1971;Acheson 1976Acheson , 1978Birg et al 1977;Treisman 1980;Treisman and Kamen 1981).…”

“…Late pre-mRNAs contain tandem copies of the noncoding late leader exon. These exons are efficiently spliced to one another, leading to the further stabilization and processing of late mRNAs Carmichael 1988, 1990;Liu and Carmichael 1993). (Bottom) RT-PCR can be used to reveal leader-to-leader splicing.…”

“…This earlylate ''switch'' is dependent on viral DNA replication; if replication is inhibited, E-RNA accumulates to abnormally high levels with minimal accumulation of L-RNA (Cogen 1978;Heiser and Eckhart 1982;Kamen et al 1982;Farmerie and Folk 1984;Hyde-DeRuyscher and Carmichael 1988). This temporally regulated switch is not controlled mainly at the level of transcription initiation; rather, it results from changes in late-strand transcription elongation and/or RNA stability (Hyde-DeRuyscher and Carmichael 1988;HydeDeRuyscher and Carmichael 1990;Liu and Carmichael 1993;Liu et al 1994). We reported earlier that late RNA accumulation is regulated post-transcriptionally (Liu and Carmichael 1993), while early gene expression is regulated by sense-antisense RNA interactions, which result in extensive adenosine-to-inosine (A-to-I) editing of earlystrand RNA molecules at late times in infection (Liu et al 1994;Kumar and Carmichael 1997).…”

“…This temporally regulated switch is not controlled mainly at the level of transcription initiation; rather, it results from changes in late-strand transcription elongation and/or RNA stability (Hyde-DeRuyscher and Carmichael 1988;HydeDeRuyscher and Carmichael 1990;Liu and Carmichael 1993;Liu et al 1994). We reported earlier that late RNA accumulation is regulated post-transcriptionally (Liu and Carmichael 1993), while early gene expression is regulated by sense-antisense RNA interactions, which result in extensive adenosine-to-inosine (A-to-I) editing of earlystrand RNA molecules at late times in infection (Liu et al 1994;Kumar and Carmichael 1997). In this study, we examine a major unsolved question regarding the polyoma life cycle: what is the trigger that initiates the early-to-late switch?…”

Induction of the Human Papillomavirus Type 31 Late Promoter Requires Differentiation but Not DNA Amplification

“…Upon differentiation in suprabasal cells, viral DNA amplification is activated, along with late transcription, leading to virion production (23). A link between productive viral replication and late gene expression has been well documented in other DNA tumor viruses such as simian virus 40 (SV40) (53,54), polyomavirus (8,9,27,31), and adenoviruses (51), but it is not clear if this extends to papillomaviruses.…”