FACT, the Bur kinase pathway, and the histone co-repressor HirC have overlapping nucleosome-related roles in yeast transcription elongation.

Gene transcription is constrained by the nucleosomal nature of chromosomal DNA. This nucleosomal barrier is modulated by FACT, a conserved histone-binding heterodimer. FACT mediates transcription-linked nucleosome disassembly and also nucleosome reassembly in the wake of the RNA polymerase II transcription complex, and in this way maintains the repression of 'cryptic' promoters found within some genes. Here we focus on a novel mutant version of the yeast FACT subunit Spt16 that supplies essential Spt16 activities but impairs transcription-linked nucleosome reassembly in dominant fashion. This Spt16 mutant protein also has genetic effects that are recessive, which we used to show that certain Spt16 activities collaborate with histone acetylation and the activities of a Bur-kinase/Spt4-Spt5/Paf1C pathway that facilitate transcription elongation. These collaborating activities were opposed by the actions of Rpd3S, a histone deacetylase that restores a repressive chromatin environment in a transcription-linked manner. Spt16 activity paralleling that of HirC, a co-repressor of histone gene expression, was also found to be opposed by Rpd3S. Our findings suggest that Spt16, the Bur/Spt4-Spt5/Paf1C pathway, and normal histone abundance and/or stoichiometry, in mutually cooperative fashion, facilitate nucleosome disassembly during transcription elongation. The recessive nature of these effects of the mutant Spt16 protein on transcription-linked nucleosome disassembly, contrasted to its dominant negative effect on transcription-linked nucleosome reassembly, indicate that mutant FACT harbouring the mutant Spt16 protein competes poorly with normal FACT at the stage of transcription-linked nucleosome disassembly, but effectively with normal FACT for transcription-linked nucleosome reassembly. This functional difference is consistent with the idea that FACT association with the transcription elongation complex depends on nucleosome disassembly, and that the same FACT molecule that associates with an elongation complex through nucleosome disassembly is retained for reassembly of the same nucleosome.

New mutant versions of yeast FACT subunit Spt16 affect cell integrity.

Transcription by RNA polymerase II is impeded by the nucleosomal organization of DNA; these negative effects are modulated at several stages of nucleosomal DNA transcription by FACT, a heterodimeric transcription factor. At promoters, FACT facilitates the binding of TATA-binding factor, while during transcription elongation FACT mediates the necessary destabilization of nucleosomes and subsequent restoration of nucleosome structure in the wake of the transcription elongation complex. Altered FACT activity can impair the fidelity of transcription initiation and affect transcription patterns. Using reporter genes we have identified new mutant versions of the Spt16 subunit of yeast FACT with dominant negative effects on the fidelity of transcription initiation. Two of these spt16 mutant alleles also affect cell integrity. Cells relying on these spt16 mutant alleles display sorbitol-remediated temperature sensitivity, altered sensitivity to detergent, and abnormal morphologies, and are further inhibited by the ssd1-d mutation. The overexpression of components of protein kinase C (Pkc1) signaling diminishes this spt16 ssd1-d temperature sensitivity, whereas gene deletions eliminating components of Pkc1 signaling further impair these spt16 mutant cells. Thus, the FACT subunit Spt16 and Pkc1 signaling have an overlapping essential function, with an unexpected role for FACT in the maintenance of cell integrity.

CD2-CD48 interactions promote interleukin-2 and interferon-gamma synthesis by stabilizing cytokine mRNA.

CD2-CD48 interactions enhance T cell receptor-driven mouse T lymphocyte activation. However, the mechanism is not well understood. Here we show that blockade of CD2-CD48 interactions with anti-CD48 monoclonal antibody (mAb) inhibited interleukin (IL)-2 and interferon (IFN)-gamma expression, as well as T cell proliferation in response to mitogenic anti-CD3 mAb, although more potent inhibition resulted from blocking CD28-CD80/CD86 interactions. Blockade of both CD2 and CD28 costimulation abrogated T cell proliferation and cytokine synthesis. Conversely, T cells stimulated with immobilized anti-CD3 and anti-CD2 mAb exhibited increased proliferation and IL-2 and IFN-gamma expression, although a stronger enhancing effect was obtained with immobilized anti-CD3 and anti-CD28 mAb. Concurrent CD2 and CD28 costimulation caused a further increase in proliferation and cytokine synthesis. Stimulation of purified T cells with microsphere-immobilized anti-CD3 and anti-CD2 mAb increased IL-2 and IFN-gamma mRNA stability. However, CD28 costimulation had a stronger enhancing effect on IL-2 and IFN-gamma mRNA stability that was not further increased by concomitant CD2 signaling. CD2, therefore, costimulates T cell activation by stabilizing cytokine mRNA transcripts, albeit with less efficiency than CD28.

Initiation-mediated mRNA decay in yeast affects heat-shock mRNAs, and works through decapping and 5'-to-3' hydrolysis.

The degradation of mRNA in the yeast Saccharomyces cerevisiae takes place through several related pathways. In the most general mRNA-decay pathway, that of poly(A)-dependent decay, the normal shortening of the poly(A) tail on an mRNA molecule by deadenylation triggers mRNA decapping by the enzyme Dcp1p, followed by exonucleolytic digestion by Xrn1p. A specialized mRNA-decay pathway, termed nonsense-mediated decay, comes into play for mRNAs that contain an early nonsense codon. This pathway operates through the Upf proteins in addition to Dcp1p and Xrn1p. Previously, we identified a different specialized mRNA-decay pathway, the initiation-mediated decay pathway, and showed that it affects two Hsp70 heat-shock mRNAs under conditions of slowed translation initiation. Here we report that initiation-mediated mRNA decay also works through the Dcp1 and Xrn1 enzymes, and requires ongoing transcription by RNA polymerase II. We show that several other heat-shock mRNAs, including two from the Hsp90 gene family and three more from the Hsp70 gene family, are also subject to initiation-mediated decay, whereas a variety of non-heat-shock mRNAs are not affected.