Stable transcription complex on a class III gene in a minichromosome

Transcriptional regulation by the immediate early protein of pseudorabies virus during in vitro nucleosome assembly

An in vivo transcriptional activator, the immediate early protein (IE) of pseudorabies virus, potentiates the activity of the major late promoter in a reconstituted chromatin assembly system where the assembly of preinitiation complexes is in competition with the assembly of promoter sequences within nucleosomes. IE function requires the simultaneous action of TFIID and results in the formation of stable preinitiation complexes within nucleosome-assembled templates. IE is unable to reverse nucleosome-mediated repression, once established, or to further increase the activity of previously activated templates. These results indicate that IE stimulates TFIID binding to promoter sequences, effectively competing with nucleosomes, during chromatin reconstitution. The specific implications for IE function in vivo and the general implications for cellular gene regulation are discussed.

Binding of transcription factor TFIID to the major late promoter during in vitro nucleosome assembly potentiates subsequent initiation by RNA polymerase II

A plasmid containing the major late promoter was assembled into nucleosomes in a Xenopus oocyte extract, isolated by gel filtration, and found to be refractory to transcription initiation in vitro. However, exposure of the promoter to HeLa nuclear extract or to a mixture of isolated transcription factors prior to nucleosome assembly prevented nucleosome-mediated repression of the promoter. Inactivation or elimination of the TATA box-binding factor (TFIID) abolished the ability of these treatments to preserve promoter function. Preincubation with TFIID alone prevented repression and resulted in TFIID being sequestered into the nucleosome-assembled templates. Preincubation with all the transcription factors resulted in the assembly of nucleosome templates containing a near complete preinitiation complex, which required only the addition of TFIIE for transcription initiation.

The presence of nucleosomes on a DNA template prevents initiation by RNA polymerase II in vitro

RNA was synthesized in vitro using HeLa cell nuclear extracts and circular DNA templates onto which varying numbers of nucleosomes had been reconstituted with Xenopus oocyte extracts. We found that fully reconstituted templates supported no specific initiation by RNA polymerase II; however, DNA exposed to the reconstitution extracts under conditions which did not allow nucleosome deposition was transcribed normally. A set of successively less reconstituted templates was also transcribed. No initiation occurred on reconstitutes with more than two-thirds of the physiological nucleosome density; reconstitutes with less than one-third of the physiological nucleosome density were transcribed as efficiently as naked DNA.

Enhancement of RNA polymerase III transcription by the E1A gene product of adenovirus

Nuclear extracts from adenovirus-infected HeLa cells harvested early in infection demonstrated a markedly increased capacity for transcription by RNA polymerase III of exogenous VA RNA genes, as well as cloned tRNA and 5S RNA genes. In contrast, no enhanced transcription was observed in extracts from cells infected with an E1A deletion mutant. Moreover, cells co-transfected with the VA- and E1A-containing plasmids showed markedly higher levels of VA RNA synthesis than did cells transfected with the VA-containing plasmid alone. Although analysis of high ionic strength extracts revealed that the enhancement of pol III transcription persists late in infection, moderate ionic strength extracts indicated that transcription factor IIIC becomes limiting. Chromatographic fractionation and complementation analysis of extracts from mock- and virus-infected cells indicated that the factor(s) responsible for the enhanced activity was localized entirely in the fraction containing transcription factor IIIC.

Formation of a rate-limiting intermediate in 5S RNA gene transcription

We have performed in vitro kinetic analyses of transcription from the 5S gene promoter in order to resolve the rate-limiting events which lead to accurate transcription of the 5S RNA gene. We demonstrate that a rate-limiting intermediate can be formed during an extended incubation prior to initiation of transcription. Formation of such a complex is temperature-dependent, requires magnesium and ATP, consists of stoichiometric amounts of the known class III transcription factors and RNA polymerase III on 5S DNA, and eliminates the normal lag in attainment of a steady-state rate of transcription. This complex is therefore different from the "stable complex" minimally required for template commitment. Further analyses demonstrate that TFIIIB, like TFIIIA and TFIIIC, can be stably sequestered on the 5S gene and also allow us to formulate the following order of factor interactions on the 5S gene: TFIIIA, TFIIIC, TFIIIB, RNA polymerase III.