Expression Of Transcription Units Research Articles

Sir2 Represses Endogenous Polymerase II Transcription Units in the Ribosomal DNA Nontranscribed Spacer

Silencing at the rDNA, HM loci, and telomeres in Saccharomyces cerevisiae requires histone-modifying enzymes to create chromatin domains that are refractory to recombination and RNA polymerase II transcription machineries. To explore how the silencing factor Sir2 regulates the composition and function of chromatin at the rDNA, the association of histones and RNA polymerase II with the rDNA was measured by chromatin immunoprecipitation. We found that Sir2 regulates not only the levels of K4-methylated histone H3 at the rDNA but also the levels of total histone H3 and RNA polymerase II. Furthermore, our results demonstrate that the ability of Sir2 to limit methylated histones at the rDNA requires its deacetylase activity. In sir2Delta cells, high levels of K4-trimethylated H3 at the rDNA nontranscribed spacer are associated with the expression of transcription units in the nontranscribed spacer by RNA polymerase II and with previously undetected alterations in chromatin structure. Together, these data suggest a model where the deacetylase activity of Sir2 prevents euchromatinization of the rDNA and silences naturally occurring intergenic transcription units whose expression has been associated with disruption of cohesion complexes and repeat amplification at the rDNA.

Expression of transcriptional units using transmissible gastroenteritis coronavirus derived minigenomes and full-length cDNA clones.

Many factors including RNA primary and secondary structure and protein-RNA interactions may regulate mRNA abundance. The nature of the transcription regulatory sequences (TRSs) and the extent of their complementarity to the leader 3’ end may be the most relevant and is discussed below. The TRSs include the core sequence (CS), previously named intergenic sequence (IG), that is a short conserved sequence element upstream the transcription units, and flanking sequences located upstream and downstream the CS. A helper dependent expression system based on transmissible gastroenteritis Coronavirus (TGEV) derived minigenomes, encoding new subgenomic mRNAs, has been used to study the elements that regulate transcription in Coronavirus. The optimization of TRSs can lead to the improvement of mRNA levels using both minigenomes and full-length cDNA clones.

PrhA controls a novel regulatory pathway required for the specific induction of Ralstonia solanacearum hrp genes in the presence of plant cells.

The Ralstonia solanacearum hrp gene cluster is organized in five transcriptional units. Expression of transcriptional units 2, 3 and 4 is induced in minimal medium and depends on the hrp regulatory gene hrpB, which belongs to unit 1. This regulatory gene also controls the expression of genes, such as popA, located to the left of the hrp cluster. Here, we show that, upon co-culture with Arabidopsis thaliana and tomato cell suspensions, the expression of the hrp transcriptional units 1, 2, 3 and 4 is induced 10- to 20-fold more than in minimal medium. This induction is not triggered by diffusible signals but requires the presence of plant cells. Moreover, we show that this specific plant cell induction of hrp genes is controlled by a gene, called prhA (plant regulator of hrp genes), located next to popA. This gene codes for a putative protein of 770 amino acids, which shows similarities with TonB-dependent outer membrane siderophore receptors. Expression of prhA and hrp genes is not regulated by iron status, and we postulate that iron is not the signal sensed by PrhA. In prhA mutants, the induction of hrpB and other hrp genes is abolished in co-culture with Arabidopsis cells, partially reduced in co-culture with tomato cells and not modified in minimal medium. prhA mutants are hypo-aggressive on Arabidopsis (accessions Col-0 and Col-5) but remain fully pathogenic on tomato plants, suggesting that the co-culture assays mimic the in planta conditions. A model suggesting that PrhA is a receptor for plant specific signals at the top of a novel hrp regulatory pathway is discussed.