P-TEFb is a key regulator of the process controlling the processivity of RNA polymerase II and possesses a kinase activity that can phosphorylate the carboxy-terminal domain of the largest subunit of RNA polymerase II. Here we report the cloning of the small subunit of Drosophila P-TEFb and the finding that it encodes a Cdc2-related protein kinase. Sequence comparison suggests that a protein with 72% identity, PITALRE, could be the human homolog of the Drosophila protein. Functional homology was suggested by transcriptional analysis of an RNA polymerase II promoter with HeLa nuclear extract depleted of PITALRE. Because the depleted extract lost the ability to produce long DRB-sensitive transcripts and this loss was reversed by the addition of purified Drosophila P-TEFb, we propose that PITALRE is a component of human P-TEFb. In addition, we found that PITALRE associated with the activation domain of HIV-1 Tat, indicating that P-TEFb is a Tat-associated kinase (TAK). An in vitro transcription assay demonstrates that the effect of Tat on transcription elongation requires P-TEFb and suggests that the enhancement of transcriptional processivity by Tat is attributable to enhanced function of P-TEFb on the HIV-1 LTR.

Summary Myocardial infarction results in compromised myocardial function with heart failure due to insufficient cardiomyocyte self-renewal1. Unlike lower vertebrates, mammalian hearts only have a transient neonatal renewal capacity2. Reactivating primitive reparative ability in the mature heart requires knowledge of the mechanisms promoting early heart repair. By testing an established Hippo-deficient heart regeneration model for renewal promoting factors, we found that Pitx2 expression was induced in injured, Hippo-deficient ventricles. Pitx2-deficient neonatal hearts failed to repair after apex resection while Pitx2-gain-of-function in adult cardiomyocytes conferred reparative ability after myocardial infarction. Genomic analyses indicated that Pitx2 activated genes encoding electron transport chain components and reactive oxygen species scavengers. A subset of Pitx2 target genes was cooperatively regulated with the Hippo effector, Yap. Furthermore, Nrf2, a regulator of antioxidant response3, directly regulated Pitx2 expression and subcellular localization. Pitx2 mutant myocardium had elevated reactive oxygen species levels while antioxidant supplementation suppressed the Pitx2-loss-of-function phenotype. These findings reveal a genetic pathway, activated by tissue damage that is essential for cardiac repair.

Pitx2 is a newly described bicoid-like homeodomain transcription factor that is defective in Rieger syndrome and shows a striking leftward developmental asymmetry. We have previously shown that Pitx2 (also called Ptx2 and RIEG) transactivates a reporter gene containing a bicoid enhancer and synergistically transactivates the prolactin promoter in the presence of the POU homeodomain protein Pit-1. In this report, we focused on the C-terminal region which is mutated in some Rieger patients and contains a highly conserved 14-amino-acid element. Deletion analysis of Pitx2 revealed that the C-terminal 39-amino-acid tail represses DNA binding activity and is required for Pitx2-Pit-1 interaction and Pit-1 synergism. Pit-1 interaction with the Pitx2 C terminus masks the inhibitory effect and promotes increased DNA binding activity. Interestingly, cotransfection of an expression vector encoding the C-terminal 39 amino acids of Pitx2 specifically inhibits Pitx2 transactivation activity. In contrast, the C-terminal 39-amino-acid peptide interacts with Pitx2 to increase its DNA binding activity. These data suggest that the C-terminal tail intrinsically inhibits the Pitx2 protein and that this inhibition can be overcome by interaction with other transcription factors to allow activation during development.

Three major PITX2 isoforms are differentially expressed in human, mice, zebrafish, chick, and frog tissues. To demonstrate differential regulation of gene expression by these isoforms we used three different promoters and three cell lines. Transient transfection of Chinese hamster ovary, HeLa, and LS-8 cell lines revealed differences in PITX2A and PITX2C activation of the PLOD1 and Dlx2 promoters, however, PITX2B is inactive. In contrast, PITX2B actives the pituitary-specific Prolactin promoter at higher levels than either PITX2A or PITX2C. Interestingly, co-transfection of either PITX2A or PITX2C with PITX2B results in a synergistic activation of the PLOD1 and Dlx2 promoters. Furthermore, PITX2 isoforms have different transcriptional activity dependent upon the cells used for transfection analysis. We have isolated a fourth PITX2 isoform (PITX2D) expressed only in humans, which acts to suppress the transcriptional activity of the other PITX2 isoforms. Electrophoretic mobility shift assays and glutathione S-transferase pull-down experiments demonstrated that all isoforms interact with PITX2D and that PITX2B forms heterodimeric complexes with PITX2A and PITX2C. Our research provides a molecular basis for differential gene regulation through the expression of PITX2 isoforms. PITX2 isoform activities are both promoter-and cell-specific, and our data reveal new mechanisms for PITX2-regulated gene expression.

Human immunodeficiency virus type 1 (HIV-1)pre-mRNA splicing is regulated in order to maintain pools of unspliced and partially spliced viral RNAs as well as the appropriate levels of multiply spliced mRNAs during virus infection. We have previously described an element in tat exon 2 that negatively regulates splicing at the upstream tat 3 splice site 3 (B. A. Amendt, D. Hesslein, L.-J. Chang, and C. M. Stoltzfus, Mol. Cell. Biol. 14:3960-3970, 1994). In this study, we further defined the element to a 20-nucleotide (nt) region which spans the C-terminal vpr and N-terminal tat coding sequences. By analogy with exon splicing enhancer (ESE) elements, we have termed this element an exon splicing silencer (ESS). We show evidence for another negative cis-acting region within tat-rev exon 3 of HIV-1 RNA that has sequence motifs in common with a 20-nt ESS element in tat exon 2. This sequence is juxtaposed to a purine-rich ESE element to form a bipartite element regulating splicing at the upstream tat-rev 3 splice site. Inhibition of the splicing of substrates containing the ESS element in tat exon 2 occurs at an early stage of spliceosome assembly. The inhibition of splicing mediated by the ESS can be specifically abrogated by the addition of competitor RNA. Our results suggest that HIV-1 RNA splicing is regulated by cellular factors that bind to positive and negative cis elements in tat exon 2 and tat-rev exon 3.Alternative splicing of mRNA precursors plays a critical role in the regulation of gene expression. In metazoan cells, splicing of pre-mRNA is mediated by cis-acting signals which include 5Ј and 3Ј splice sites, branchpoint sequences, and polypyrimidine tracts preceding 3Ј splice sites (for a review, see reference 19). However, the mechanisms by which alternative splice site selection is regulated are not well understood. There are numerous examples of sequences within introns that act to either enhance or inhibit splicing (3,7,10,16,21,25,35,40,43,65). Some of these intron sequences have been shown to bind cellular factors (21,35,40,43). Exon sequences have also been shown to play a role in alternative splicing. Positive-acting exon sequences and purine-rich regions or exon splicing enhancer (ESE) elements have been reported for a number of different cellular and viral genes (4,5,8,23,30,36,50,51,53,56,(58)(59)(60). Some of these positive-acting exon sequences are binding sites for cellular factors (5,23,30,50,58). A family of factors called SR proteins are required for splicing and, in some cases, have been shown to regulate alternative splice site selection in a concentration-dependent manner (14,17,28,33,61). Recent reports have shown that the SR proteins selectively bind to purine-rich splicing elements present in cellular exons (30,49,50). There are also several examples of negative-acting exon splicing elements (2,4,18,45,55). To date, factors interacting with negative-acting exon splicing elements affecting alternative 3Ј splice site usage in metazoan cells have not yet been reported.Human immunodeficie...