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.

The mechanism of replication of the simian virus 40 (SV40) genome closely resembles that of cellular chromosomes, thereby providing an excellent model system for examining the enzymatic requirements for DNA replication. Only one viral gene product, the large tumour antigen (large-T antigen), is required for viral replication, so the majority of replication enzymes must be cellular. Indeed, a number of enzymatic activities associated with replication and the S phase of the cell cycle are induced upon SV40 infection. Cell-free extracts derived from human cells, when supplemented with immunopurified SV40 large-T antigen support efficient replication of plasmids that contain the SV40 origin of DNA replication. Using this system, a cellular protein of relative molecular mass 36,000 (Mr = 36K) that is required for the elongation stage of SV40 DNA replication in vitro has been purified and identified as a known cell-cycle regulated protein, alternatively called the proliferating cell nuclear antigen (PCNA) or cyclin. It was noticed that, in its physical characteristics, PCNA closely resembles a protein that regulates the activity of calf thymus DNA polymerase-delta. Here we show that PCNA and the polymerase-delta auxiliary protein have similar electrophoretic behaviour and are both recognized by anti-PCNA human autoantibodies. More importantly, both proteins are functionally equivalent; they stimulate SV40 DNA replication in vitro and increase the processivity of calf thymus DNA polymerase-delta. These results implicate a novel animal cell DNA polymerase, DNA polymerase-delta, in the elongation stage of replicative DNA synthesis in vitro.

No abstract

The interferon-induced protein kinase DAI, the double-stranded RNA (dsRNA)-activated inhibitor of translation, plays a key role in regulating protein synthesis in higher cells. Once activated, in a process that involves autophosphorylation, it phosphorylates the initiation factor eIF-2, leading to inhibition of polypeptide chain initiation. The In mammalian cells, a regulatory mechanism involving an RNA-activated protein kinase and the eukaryotic initiation factor 2 (eIF-2) has been intensively studied. This initiation factor forms a ternary complex with GTP and Met-tRNAF and delivers the initiator tRNA to the ribosomal site of protein synthesis initiation. Discharged eIF-2 is subsequently released as a complex with GDP which must be replaced with GTP to permit the formation of another ternary complex in preparation for a further round of initiation. The factor is composed of three dissimilar subunits, cx, I, and -y. Phosphorylation of a single residue, serine-51 of the a subunit, inhibits translation by trapping a second initiation factor, the guanosine nucleotide exchange factor (or eIF-2B), which is required to catalyze the substitution of GTP for GDP in the discharged eIF-2 complex. Phosphorylation of sufficient eIF-2 can sequester all of the guanosine nucleotide exchange factor, thereby preventing eIF-2 recycling and halting the initiation pathway.In mammals, two protein kinases are capable of phosphorylating the a subunit of eIF-2 in this way (reviewed in references 20, 37, and 46). One of them, the heme-controlled repressor, is found chiefly in reticulocytes. It is activated by the absence of hemin, as well as by other stimuli, and serves to prevent the accumulation of globin in the absence of iron or heme. A second kinase, the double-stranded RNA-activated inhibitor (DAI; also referred to as P1 kinase, p68 kinase, P1/eIF-2a kinase, and PKdS, etc.) is present in a wide range of tissues. DAI is an important element in the host antiviral response, and its synthesis is induced at the transcriptional level by interferon (reviewed in references 21, 54, 56, and 59). The enzyme is ribosome associated (11,34) (8,24,26,36,47,60). It has also been implicated in cellular differentiation (23,52), in the inhibition of cell proliferation (6, 51), in the heat shock response (10), and possibly in transcriptional induction (61, 64). Moreover, in yeast cells, the related protein kinase GCN2 mediates the growth response to amino acid starvation (9). As its name implies, DAI is activated by doublestranded RNA (dsRNA). Other polyanions such as heparin can also activate it, while small, highly structured RNA molecules such as adenovirus VA RNA suppress its activation (38). Thus, DAI is a pivotal cellular regulatory enzyme whose level and activity are modulated by factors of both viral and cellular origin.The interactions between DAI and its RNA effectors are complicated and incompletely understood. The kinase is activated by dsRNA but not by DNA or DNA-RNA hybrids (22,32,35,58). Single-stranded RNA, either synthetic or natur...

No abstract