A 21-base pair DNA fragment directs transcription attenuation within the simian virus 40 late leader

H2-M3 restricted presentation of a Listeria-derived leader peptide

Protective immunity to infection by many intracellular pathogens requires recognition by cytotoxic T lymphocytes (CTLs) of antigens presented on major histocompatibility complex (MHC) class I molecules. To be presented for recognition by pathogen-specific CTLs, these antigens must gain access to the host cell class I processing pathway. In the case of intracellular bacterial pathogens, the majority of bacterial proteins are retained within the bacterial membrane and therefore remain inaccessible to the host cell for antigen processing. We have isolated a CTL clone from a C57BL/6 mouse infected with the intracellular gram-positive bacterium Listeria monocytogenes (LM) and have identified the source of the antigen. Using a genomic expression library, we determined that the clone recognizes an antigenic N-formyl peptide presented by the nonpolymorphic murine MHC class Ib molecule, H2-M3. Several lengths of this peptide were able to sensitize cells for lysis by this CTL clone. The source of this antigenic peptide is a 23-amino acid polypeptide encoded at the start of a polycistronic region. Analysis of mRNA secondary structure of this region suggests that this polypeptide may be a leader peptide encoded by a transcriptional attenuator.

Control of RNA polymerase II elongation potential by a novel carboxyl-terminal domain kinase

The entry of RNA polymerase II into a productive mode of elongation is controlled, in part, by the postinitiation activity of positive transcription elongation factor b (P-TEFb) (Marshall, N. F., and Price, D. H. (1995) J. Biol. Chem. 270, 12335-12338). We report here that removal of the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase II abolishes productive elongation. Correspondingly, we found that P-TEFb can phosphorylate the CTD of pure RNA polymerase II. Furthermore, P-TEFb can phosphorylate the CTD of RNA polymerase II when the polymerase is in an early elongation complex. Both the function and kinase activity of P-TEFb are blocked by the drugs 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and H-8. P-TEFb is distinct from transcription factor IIH (TFIIH) because the two factors have no subunits in common, P-TEFb is more sensitive to DRB than is TFIIH, and most importantly, TFIIH cannot substitute functionally for P-TEFb. We propose that phosphorylation of the CTD by P-TEFb controls the transition from abortive into productive elongation mode.

Purification of P-TEFb, a transcription factor required for the transition into productive elongation

Production of full-length runoff transcripts in vitro and functional mRNA in vivo is sensitive to the drug 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). We previously proposed the existence of an activity, P-TEF (positive transcription elongation factor) that functions in a DRB-sensitive manner to allow RNA polymerase II elongation complexes to efficiently synthesize long transcripts (Marshall, N. F. and Price, D. H. (1992) Mol. Cell. Biol. 12, 2078-2090). We have fractionated nuclear extracts of Drosophila melanogaster Kc cells and identified three activities, P-TEFa, factor 2, and P-TEFb, that are directly involved in reconstructing DRB-sensitive transcription. P-TEFb is essential for the production of DRB-sensitive long transcripts in vitro, while P-TEFa and factor 2 are stimulatory. P-TEFb activity is associated with a protein comprising two polypeptide subunits with apparent molecular masses of 124 and 43 kDa. Using a P-TEFb-dependent transcription system, we show that P-TEFb acts after initiation and is the limiting factor in the production of long run-off transcripts.

Parameters affecting the elongation block by RNA polymerase II at the SV40 attenuator 1 in vitro

We have previously suggested that folding of the SV40 attenuator RNA 1 into two hairpin elements leads to a block of transcription elongation. Using site-directed mutagenesis, we created three templates that either strengthened or weakened the proposed hairpin structures. The mutated and wild-type templates were cloned downstream from the adenovirus 2 MLP, and transcription patterns were compared among the templates, in cell-free extracts. In this report, we show that at the standard temperature (30 degrees C) the elongation block occurs at elevated KCl concentrations (0.5-1.2 M KCl) while at high temperature (65 degrees C), although the transcription elongation rate increases, the block to elongation occurs at lower KCl concentrations (less than 0.5 M KCl) as well. Since under the conditions tested the extent of the elongation block is directly dependent on the stability of the hairpin structure of the RNA, as assessed by a comparison of transcription of the wild-type and mutated templates, we suggest that elevated KCl concentrations and high temperatures are favored conditions for optimizing the processes whereby the hairpin structures are recognized by the polymerase as an attenuation signal. Moreover, the present experiments indicate that cellular elongation factors are not directly involved in modulating the extent of the elongation block at the SV40 attenuator 1 in vitro. The SV40 attenuator 1 is the only eukaryotic system known to date which has been shown to have structural characteristics so similar to rho-independent terminator in prokaryotes. We discuss the similarities between the mechanisms which lead to elongation blocks at these eukaryotic and prokaryotic sites.