promSR34a::GUS Introduction SR proteins constitute a highly conserved family of non-snRNP splicing factors (1). They have many functions in splicing as they participate in spliceosome assembly and in alternative splicing by influencing the splice site selection. In addition, they also have a role in post-splicing events such as mRNA export or translation efficiency. SR proteins have at least one N-terminal RNA-binding domain (RRM) and a C-terminal RS domain enriched in serine/arginine dipeptides (2). We have used different approaches to study gene expression patterns and dynamic localizations of ASF/SF2-like proteins in Arabidopsis thaliana.
Abstract Serine/arginine-rich (SR) proteins are splicing regulators that share a modular structure consisting of one or two N-terminal RNA recognition motif domains and a C-terminal RS-rich domain. We investigated the dynamic localization of the Arabidopsis thaliana SR protein RSZp22, which, as we showed previously, distributes in predominant speckle-like structures and in the nucleolus. To determine the role of RSZp22 diverse domains in its nucleolar distribution, we investigated the subnuclear localization of domain-deleted mutant proteins. Our results suggest that the nucleolar localization of RSZp22 does not depend on a single targeting signal but likely involves different domains/motifs. Photobleaching experiments demonstrated the unrestricted dynamics of RSZp22 between nuclear compartments. Selective inhibitor experiments of ongoing cellular phosphorylation influenced the rates of exchange of RSZp22 between the different nuclear territories, indicating that SR protein mobility is dependent on the phosphorylation state of the cell. Furthermore, based on a leptomycin B– and fluorescence loss in photobleaching–based sensitive assay, we suggest that RSZp22 is a nucleocytoplasmic shuttling protein. Finally, with electron microscopy, we confirmed that RSp31, a plant-specific SR protein, is dynamically distributed in nucleolar cap-like structures upon phosphorylation inhibition. Our findings emphasize the high mobility of Arabidopsis SR splicing factors and provide insights into the dynamic relationships between the different nuclear compartments.
Additional information on the morphology of the vegetative ultimate branches of Archaeopteris roemeriana was obtained by uncovering compression specimens collected in Upper Devonian deposits in Belgium. For the first time, anisophylly, previously inferred from anatomical studies, is demonstrated on one species of the genus. Small leaves less than half the size of those more readily seen in unprepared compressions are borne on the adaxial surface of the ultimate branches. The phyllotaxis is discussed and the eventual adaptation to light interception of the combination of anisophylly and shoot dorsiventrality is put forward.
