Molecular Cell

Spt5 is an essential and conserved factor that functions in transcription and co-transcriptional processes. However, many aspects of the requirement for Spt5 in transcription are poorly understood. We have analyzed the consequences of Spt5 depletion in Schizosaccharomyces pombe using four genome-wide approaches. Our results demonstrate that Spt5 is crucial for a normal rate of RNA synthesis and distribution of RNAPII over transcription units. In the absence of Spt5, RNAPII localization changes dramatically, with reduced levels and a relative accumulation over the first ∼500 bp, suggesting that Spt5 is required for transcription past a barrier. Spt5 depletion also results in widespread antisense transcription initiating within this barrier region. Deletions of this region alter the distribution of RNAPII on the sense strand, suggesting that the barrier observed after Spt5 depletion is normally a site at which Spt5 stimulates elongation. Our results reveal a global requirement for Spt5 in transcription elongation.

Variation in chromatin composition and organization often reflects differences in genome function. Histone variants, for example, replace canonical histones to contribute to regulation of numerous nuclear processes including transcription, DNA repair, and chromosome segregation. Here we focus on H2A.Bbd, a rapidly evolving variant found in mammals but not in invertebrates. We report that in human cells, nucleosomes bearing H2A.Bbd form unconventional chromatin structures enriched within actively transcribed genes and characterized by shorter DNA protection and nucleosome spacing. Analysis of transcriptional profiles from cells depleted for H2A.Bbd demonstrated widespread changes in gene expression with a net downregulation of transcription and disruption of normal mRNA splicing patterns. In particular, we observed changes in exon inclusion rates and increased presence of intronic sequences in mRNA products upon H2A.Bbd depletion. Taken together, our results indicate that H2A.Bbd is involved in formation of a specific chromatin structure that facilitates both transcription and initial mRNA processing.

In higher eukaryotes, up to 70% of genes have high levels of nonmethylated cytosine/guanine base pairs (CpGs) surrounding promoters and gene regulatory units. These features, called CpG islands, were identified over 20 years ago, but there remains little mechanistic evidence to suggest how these enigmatic elements contribute to promoter function, except that they are refractory to epigenetic silencing by DNA methylation. Here we show that CpG islands directly recruit the H3K36-specific lysine demethylase enzyme KDM2A. Nucleation of KDM2A at these elements results in removal of H3K36 methylation, creating CpG island chromatin that is uniquely depleted of this modification. KDM2A utilizes a zinc finger CxxC (ZF-CxxC) domain that preferentially recognizes nonmethylated CpG DNA, and binding is blocked when the CpG DNA is methylated, thus constraining KDM2A to nonmethylated CpG islands. These data expose a straightforward mechanism through which KDM2A delineates a unique architecture that differentiates CpG island chromatin from bulk chromatin.