No abstract

RNA polymerase transcription initiation sites are largely unknown in Caenorhabditis elegans. The initial 59 end of most protein-coding transcripts is removed by trans-splicing, and noncoding initiation sites have not been investigated. We characterized the landscape of RNA Pol II transcription initiation, identifying 73,500 distinct clusters of initiation. Bidirectional transcription is frequent, with a peak of transcriptional pairing at 120 bp. We assign transcription initiation sites to 7691 protein-coding genes and find that they display features typical of eukaryotic promoters. Strikingly, the majority of initiation events occur in regions with enhancer-like chromatin signatures. Based on the overlap of transcription initiation clusters with mapped transcription factor binding sites, we define 2361 transcribed intergenic enhancers. Remarkably, productive transcription elongation across these enhancers is predominantly in the same orientation as that of the nearest downstream gene. Directed elongation from an upstream enhancer toward a downstream gene could potentially deliver RNA polymerase II to a proximal promoter, or alternatively might function directly as a distal promoter. Our results provide a new resource to investigate transcription regulation in metazoans.

Paternal contributions to epigenetic inheritance are not well understood. Paternal contributions via marked nucleosomes are particularly understudied, in part because sperm in some organisms replace the majority of nucleosome packaging with protamine packaging. Here we report that in Caenorhabditis elegans sperm, the genome is packaged in nucleosomes and carries a histone-based epigenetic memory of genes expressed during spermatogenesis, which unexpectedly include genes well known for their expression during oogenesis. In sperm, genes with spermatogenesis-restricted expression are uniquely marked with both active and repressive marks, which may reflect a sperm-specific chromatin signature. We further demonstrate that epigenetic information provided by sperm is important and in fact sufficient to guide proper germ cell development in offspring. This study establishes one mode of paternal epigenetic inheritance and offers a potential mechanism for how the life experiences of fathers may impact the development and health of their descendants.

Nucleosomes have structural and regulatory functions in all eukaryotic DNA-templated processes. The position of nucleosomes on DNA and the stability of the underlying histone-DNA interactions affect the access of regulatory proteins to DNA. Both stability and position are regulated through DNA sequence, histone post-translational modifications, histone variants, chromatin remodelers, and transcription factors. Here, we explored the functional implications of nucleosome properties on gene expression and development in Caenorhabditis elegans embryos. We performed a time-course of micrococcal nuclease (MNase) digestion and measured the relative sensitivity or resistance of nucleosomes throughout the genome. Fragile nucleosomes were defined by nucleosomal DNA fragments that were recovered preferentially in early MNase-digestion time points. Nucleosome fragility was strongly and positively correlated with the AT content of the underlying DNA sequence. There was no correlation between promoter nucleosome fragility and the levels of histone modifications or histone variants. Genes with fragile nucleosomes in their promoters tended to be lowly expressed and expressed in a contextspecific way, operating in neuronal response, the immune system, and stress response. In addition to DNA-encoded nucleosome fragility, we also found fragile nucleosomes at locations where we expected to find destabilized nucleosomes, for example, at transcription factor binding sites where nucleosomes compete with DNA-binding factors. Our data suggest that in C. elegans promoters, nucleosome fragility is in large part DNA-encoded and that it poises genes for future context-specific activation in response to environmental stress and developmental cues.[Supplemental material is available for this article.]The fundamental unit of eukaryotic chromatin is the nucleosome, which consists of 147 bp of DNA wrapped around an octamer of histone proteins (Luger et al. 1997). Nucleosomes have important structural and regulatory functions in organizing the genome and restricting access of regulatory factors to the DNA sequence (Henikoff 2008). As such, the interactions between nucleosomes and DNA strongly influence the regulation of gene expression by determining DNA accessibility for transcription factors (TFs) and RNA polymerase. In addition to regulated nucleosome assembly and disassembly through the action of histone chaperones and chromatin remodelers, nucleosome stability is influenced by histone modifications, histone variants, DNA features encoded in cis, and competition with DNA-binding factors in trans . A complete picture of the mechanisms governing nucleosome stability is fundamental to understanding how gene expression is dynamically regulated.Nucleosome stability has been studied in vitro using sensitivity to enzymatic digestion or salt concentration (Bloom and Anderson 1978;Burton et al. 1978;Li et al. 1993;Polach and Widom 1995;Wu and Travers 2004;Jin and Felsenfeld 2007). Genome-wide adaptations of these methods have been used to identify nuc...

17There was not a strong correlation between promoter nucleosome fragility and the levels of 18 histone modifications or histone variants. Our data suggest that in C. elegans promoters,