Arabidopsis mRNA decay landscape arises from specialized RNA decay substrates, decapping-mediated feedback, and redundancy.

The decay of mRNA plays a vital role in modulating mRNA abundance, which, in turn, influences cellular and organismal processes. In plants and metazoans, three distinct pathways carry out the decay of most cytoplasmic mRNAs: The mRNA decapping complex, which requires the scaffold protein VARICOSE (VCS), removes a protective 5' cap, allowing for 5' to 3' decay via EXORIBONUCLEASE4 (XRN4, XRN1 in metazoans and yeast), and both the exosome and SUPPRESSOR OF VCS (SOV)/DIS3L2 degrade RNAs in the 3' to 5' direction. However, the unique biological contributions of these three pathways, and whether they degrade specialized sets of transcripts, are unknown. In Arabidopsis, the participation of SOV in RNA homeostasis is also unclear, because Arabidopsis sov mutants have a normal phenotype. We carried out mRNA decay analyses in wild-type, sov, vcs, and vcs sov seedlings, and used a mathematical modeling approach to determine decay rates and quantify gene-specific contributions of VCS and SOV to decay. This analysis revealed that VCS (decapping) contributes to decay of 68% of the transcriptome, and, while it initiates degradation of mRNAs with a wide range of decay rates, it especially contributes to decay of short-lived RNAs. Only a few RNAs were clear SOV substrates in that they decayed more slowly in sov mutants. However, 4,506 RNAs showed VCS-dependent feedback in sov that modulated decay rates, and, by inference, transcription, to maintain RNA abundances, suggesting that these RNAs might also be SOV substrates. This feedback was shown to be independent of siRNA activity.

Early stages of induction of anterior head ectodermal properties in Xenopus embryos are mediated by transcriptional cofactor ldb1.

BACKGROUND: Specific molecules involved in early inductive signaling from anterior neural tissue to the placodal ectoderm to establish a lens-forming bias, as well as their regulatory factors, remain largely unknown. In this study, we sought to identify and characterize these molecules. RESULTS: Using an expression cloning strategy to isolate genes with lens-inducing activity, we identified the transcriptional cofactor ldb1. This, together with evidence for its nuclear dependence, suggests its role as a regulatory factor, not a direct signaling molecule. We propose that ldb1 mediates induction of early lens genes in our functional assay by transcriptional activation of lens-inducing signals. Gain-of-function assays demonstrate that the inductive activity of the anterior neural plate on head ectodermal structures can be augmented by ldb1. Loss-of-function assays show that knockdown of ldb1 leads to decreased expression of early lens and retinal markers and subsequently to defects in eye development. CONCLUSIONS: The functional cloning, expression pattern, overexpression, and knockdown data show that an ldb1-regulated mechanism acts as an early signal for Xenopus lens induction.