VP16 and ubiquitin: Binding of P-TEFb via its activation domain and ubiquitin facilitates elongation of transcription of target genes

Acidic or type IIB transcriptional activation domains (AADs) increase rates of initiation as well as elongation of transcription [1]. For the former effects, AADs bind general transcription factors and larger coactivator complexes, which position RNA polymerase II(RNAPII) at sites of initiation of transcription [2]. For the latter effects, their ubiquitylation plays an important role [3]. In this study, this posttranslational modification increased the binding between a prototypic AAD and the positive transcription elongation factor b (P-TEFb), which contains a C-type cyclin (CycT1, CycT2, or CycK) and Cdk9 [4]. By phosphorylating negative elongation factors and the C-terminal domain of RNAPII, P-TEFb modifies the transcription complex for efficient elongation and cotranscriptional processing of mRNA [5, 6]. Indeed, the activation domain of VP16 and ubiquitin bound the cyclin boxes and the C terminus in CycT1, respectively. Moreover, the artificial fusion of ubiquitin with VP16 not only increased its activity via DNA and RNA, which was reflected in increased ratios of elongated to initiated transcripts, but rescued the deleterious substitution of alanine for phenylalanine at position 442 in its AAD. Thus, the ubiquitylation of AADs increases their interaction with P-TEFb and augments rates of elongation of transcription.