ABSTRACT
The cohesin complex is a key player in regulating cell division. Cohesin proteins SMC1, SMC3, Rad21, and stromalin (SA), along with associated proteins Nipped-B, Pds5, and EcoI, maintain sister chromatid cohesion before segregation to daughter cells during anaphase. Recent chromatin immunoprecipitation (ChIP) data reveal extensive overlap of Nipped-B and cohesin components with RNA polymerase II binding at active genes in Drosophila. These and other data strongly suggest a role for cohesion in transcription; however, there is no clear evidence for any specific mechanisms by which cohesin and associated proteins regulate transcription. We report here a link between cohesin components and trithorax group (trxG) function, thus implicating these proteins in transcription activation and/or elongation. We show that the Drosophila Rad21 protein is encoded by verthandi (vtd), a member of the trxG gene family that is also involved in regulating the hedgehog (hh) gene. In addition, mutations in the associated protein Nipped-B show similar trxG activity i.e., like vtd, they act as dominant suppressors of Pc and hh(Mrt) without impairing cell division. Our results provide a framework to further investigate how cohesin and associated components might regulate transcription.
Subject(s)
Cell Cycle Proteins/chemistry , Cell Cycle Proteins/physiology , Chromosomal Proteins, Non-Histone/chemistry , Chromosomal Proteins, Non-Histone/physiology , Drosophila Proteins/physiology , Transcription, Genetic , Animals , Cell Cycle Proteins/classification , DNA-Binding Proteins/genetics , Drosophila , Drosophila Proteins/classification , Drosophila Proteins/genetics , Embryo, Nonmammalian , Gene Expression Regulation , CohesinsABSTRACT
BACKGROUND: Histone protein synthesis is essential for cell proliferation and required for the packaging of DNA into chromatin. In animals, histone proteins are provided by the expression of multicopy replication-dependent histone genes. Histone mRNAs that are processed by a histone-specific mechanism to end after a highly conserved RNA hairpin element, and lack a poly(A) tail. In vertebrates and Drosophila, their expression is dependent on HBP/SLBP that binds to the RNA hairpin element. We showed previously that these cis and trans acting regulators of histone gene expression are conserved in C. elegans. Here we report the results of an investigation of the histone mRNA 3' end structure and of histone gene expression during C. elegans development. RESULTS: Sequence analysis of replication-dependent histone genes revealed the presence of several highly conserved sequence elements in the 3' untranslated region of histone pre-mRNAs, including an RNA hairpin element and a polyadenylation signal. To determine whether in C. elegans histone mRNA 3' end formation occurs at this polyadenylation signal and results in polyadenylated histone mRNA, we investigated the mRNA 3' end structure of histone mRNA. Using poly(A) selection, RNAse protection and sequencing of histone mRNA ends, we determined that a majority of C. elegans histone mRNAs lack a poly(A) tail and end three to six nucleotides after the hairpin structure, after an A or a U, and have a 3' OH group. RNAi knock down of CDL-1, the C. elegans HBP/SLBP, does not significantly affect histone mRNA levels but severely depletes histone protein levels. Histone gene expression varies during development and is reduced in L3 animals compared to L1 animals and adults. In adults, histone gene expression is restricted to the germ line, where cell division occurs. CONCLUSION: Our findings indicate that the expression of C. elegans histone genes is subject to control mechanisms similar to the ones in other animals: the structure of C. elegans histone mRNA 3' ends is compatible with histone-specific mRNA 3' end processing; CDL-1 functions in post-transcriptional control of histone gene expression; and C. elegans histone mRNA levels are elevated at periods of active cell division, indicating that histone gene expression is linked to DNA replication.
