In the PNAS Early Edition this week, researchers at the University of North Carolina, Chapel Hill, and the National Cancer Institute describe how, through their investigation of the structure of murine leukemia virus RNA, they identified a "high-affinity Gag recognition structure" that mediates the packaging of the retroviral genome. Specifically, NC, which is derived from Gag, "binds within the virus to the sequence UCUG-UR-UCUG," the authors write, adding that structural context is critical since "mutating the guanosine residues ... reduced packaging 100-fold."
Investigators at the University of Toronto provide structural evidence for how the polycomb repressor complex 2 binds H3K27me3 to preserve histone methylation patterns during replication in PNAS this week. Specifically, they report data that "provide an explanation for the preferential recognition of the Ala-Arg-Lys-Ser motif-containing trimethylated H3K27, H3K9, and H1K26 marks by EED [a PRC2 component] over lower methylation states and other histone methyllysine marks," they write. In addition, the Toronto team found that "binding of different histone marks by EED differentially regulates the activity and specificity of PRC2," which explains, in part, how histone marks can regulate histone methyltransferase activity.
A team led by researchers at the Oregon Health and Science University used a novel affinity purification approach to find that miR-133a binds the mRNA encoding Hand2, an essential cardiac development transcription factor. The group proposes that Hand2 is regulated by miR-133a and miR-1, which is known to also bind the transcription factor.
In another PNAS paper published online in advance, researchers in Japan show that the transcriptional co-repressor Xenopus furry "contributes to release of microRNA gene silencing." While the N-terminal domain of fuzzy acts as a repressor, the C-terminal leucine zipper, found only in vertebrate homologs, plays a role in nuclear localization, the team writes. In their investigation of Xfurry transcriptional targets, the researchers found that the co-repressor "regulates chordamesodermal genes by contributing to repression of pre-transcriptional gene silencing by miR-15."