The RNA polymerase II core promoter is a structurally and functionally diverse transcriptional regulatory element. There are two main strategies for transcription initiation – focused and dispersed initiation. In focused initiation, transcription starts from a single nucleotide or within a cluster of several nucleotides, whereas in dispersed initiation, there are several weak transcription start sites over a broad region of about 50 to 100 nucleotides. Focused initiation is the predominant means of transcription in simpler organisms, whereas dispersed initiation is observed in approximately two thirds of vertebrate genes. Regulated genes tend to have focused promoters, and constitutive genes typically have dispersed promoters. Hence, in vertebrates, focused promoters are used in a small but biologically important fraction of genes. The properties of focused core promoters are dependent upon the presence or absence of sequence motifs such as the TATA box and DPE. For example, Caudal, a key regulator of the homeotic gene network, preferentially activates transcription from DPE- versus TATA-dependent promoters. The basal transcription factors, which act in conjunction with the core promoter, are another important component in the regulation of gene expression. For instance, upon differentiation of myoblasts to myotubes, the cells undergo a switch from a TFIID-based transcription system to a TRF3-TAF3-based system. These findings suggest that the core promoter and basal transcription factors are important yet mostly unexplored components in the regulation of gene expression.

SummaryThe RNA polymerase II core promoter is generally defined to be the sequence that directs the initiation of transcription. This simple definition belies a diverse and complex transcriptional module. There are two major types of core promoters -focused and dispersed. Focused promoters contain either a single transcription start site or a distinct cluster of start sites over several nucleotides, whereas dispersed promoters contain several start sites over 50 to 100 nucleotides and are typically found in CpG islands in vertebrates. Focused promoters are more ancient and widespread throughout nature than dispersed promoters; however, in vertebrates, dispersed promoters are more common than focused promoters. In addition, core promoters may contain many different sequence motifs, such as the TATA box, BRE, Inr, MTE, DPE, DCE, and XCPE1, that specify different mechanisms of transcription and responses to enhancers. Thus, the core promoter is a sophisticated gateway to transcription that determines which signals will lead to transcription initiation.

The mdm2 proto-oncogene product binds to the p53 tumor suppressor protein and inhibits its ability to trans-activate target genes. One such target gene is mdm2 itself, which is therefore considered a component of a p53 negative feedback loop. Two tandem p53-binding motifs residing within the first intron of the murine mdm2 gene confer upon it p53-mediated activation. We now report that in murine cells p53 activates an internal mdm2 promoter (P2) located near the 3' end of intron 1, resulting in mRNA whose transcription starts within exon 2. P2 is activated by p53 within artificial constructs, as well as within the context of the chromosomal mdm2 gene. Activation follows either the introduction of overexpressed wild-type p53 into cells or the induction of endogenous wild-type p53 by ionizing radiation. The upstream, constitutive (P1) mdm2 promoter is only mildly affected by p53, if at all. The p53-derived mdm2 transcripts lack exon 1 and a few nucleotides from exon 2. As the first in-frame AUG of mdm2 is located within exon 3, the two types of mdm2 transcripts should possess similar coding potentials. Nevertheless, in vitro conditions, where each of these transcripts yields a distinct translation profile, reflect the differential usage of translation initiation codons. Initiation of translation at internal AUG codons, which occurs also in vivo, gives rise to MDM2 polypeptides incapable of binding to p53. In vitro translation profiles of the various mdm2 transcripts could be manipulated by changing the amounts of input RNA. Thus, p53 can modulate both the amount and the nature of MDM2 polypeptides through activation of the internal P2 promoter.

contributed equally to this workIn response to environmental stress, the p53 phosphoprotein is stabilized and activated to inhibit cell growth. p53 stability and activity are negatively regulated by the murine double minute (Mdm2) oncoprotein in an autoregulatory feedback loop. The inhibitory effect of Mdm2 on p53 has to be tightly regulated for proper p53 activity. Phosphorylation is an important level of p53 regulation. In response to DNA damage, p53 is phosphorylated at several N-terminal serines. In this study we examined the role of Ser20, a potential phosphorylation site in human p53, in the regulation of p53 stability and function. Substitution of Ser20 by Ala (p53-Ala20) significantly increases the susceptibility of human p53 to negative regulation by Mdm2 in vivo, as measured by apoptosis and transcription activation assays. Mutation of Ser20 to Ala renders p53 less stable and more prone to Mdm2-mediated degradation. While the in vitro binding of p53 to Mdm2 is not increased by the Ala20 mutation, the same mutation results in a markedly enhanced binding in vivo. This is consistent with the conclusion that phosphorylation of Ser20 in vivo attenuates the binding of wild-type p53 to Mdm2. Peptides bearing nonphosphorylated Ser20 or Ala20 compete with p53 for Mdm2 binding, while a similar peptide with phosphorylated Ser20 does not. This implies a critical role for Ser20 in modulating the negative regulation of p53 by Mdm2, probably through phosphorylationdependent inhibition of p53-Mdm2 interaction.

Transcription is a critical component in the expression of genes. Here we describe the design and analysis of a potent core promoter, termed super core promoter 1 (SCP1), which directs high amounts of transcription by RNA polymerase II in metazoans. SCP1 contains four core promoter motifs-the TATA box, initiator (Inr), motif ten element (MTE) and downstream promoter element (DPE)-in a single promoter, and is distinctly stronger than the cytomegalovirus (CMV) IE1 and adenovirus major late (AdML) core promoters both in vitro and in vivo. Each of the four core promoter motifs is needed for full SCP1 activity. SCP1 is bound efficiently by TFIID and exhibits a high propensity to form productive transcription complexes. SCP1 and related super core promoters (SCPs) with multiple core promoter motifs will be useful for the biophysical analysis of TFIID binding to DNA, the biochemical investigation of the transcription process and the enhancement of gene expression in cells.