Book of Abstracts: Albany 2005
Nonequivalent Functions of the Individual Exosome Subunits and an Unexpected Critical Role of Exosome in the Reproductive Development in Arabidopsis
Exosome is an evolutionarily ancient macromolecular machine that is composed of multiple hydrolytic as well as phosphorolytic 3?-5? exoribonucleases and additional, auxiliary factors. Exosome plays a key role in the numerous, mechanistically distinct reactions of RNA processing, proofreading and degradation that occur in the nucleus as well as in the cytoplasm. A structural model of the exosome envisions that its phosphorolytic subunits make greater contribution towards the assembly and/or integrity of the complex compared to the hydrolytic subunits. However, there is little direct evidence to support this view, because every exosome subunit in yeast, except the nuclear-specific RRP6, is essential, and the phenotypes elicited by genetic depletion or partial loss of function mutations of all other subunits are similar. Therefore, an alternative view holds that a complete, intact complex is required for all exosome-mediated reactions and cell viability, and that the removal or inactivation of any individual subunit completely inactivates the whole complex (?house of cards? model).
To date, the effect of the loss of the exosome, or its individual subunits has not been investigated at the whole-organism level in any of the multicellular species of life. In this paper, we describe the purification of the Arabidopsis thaliana exosome complex and identification of its subunits. In addition, a novel genetic depletion system targeting the RRP4 and RRP41 Arabidopsis exosome subunits via inducible RNA interference (iRNAi) has been established, and allowed to reveal the characteristic defects in the ribosomal RNA processing, as well as novel effects of the exosome on the RNAP III transcripts. Furthermore, a detailed analysis of the knockout phenotypes of the two of the Arabidopsis exosome subunits quite unexpectedly revealed their specific and distinct functions in reproductive development, including the embryo and endosperm identity specification, parental imprinting and development of the female gametophyte. This is the first direct evidence that the individual subunits of the exosome core are functionally specialized.
Sergei V. Reverdatto1,a
1Department of Biological Sciences