Translation of animal virus RNA in the cytoplasm of a plant cell

A cell-free system for light-dependent nuclear import of phytochrome

Translocation from the cytosol to the nucleus is an essential step in phytochrome (phy) signal transduction. In the case of phytochrome A (phyA), this step occurs with the help of FHY1 (far-red-elongated hypocotyl 1), a specific transport protein. To investigate the components involved in phyA transport, we used a cell-free system that facilitates the controlled addition of transport factors. For this purpose, we isolated nuclei from the unicellular green algae Acetabularia acetabulum. These nuclei are up to 100 mum in diameter and allow easy detection of imported proteins. Experiments with isolated nuclei of Acetabularia showed that FHY1 is sufficient for phyA transport. The reconstituted system demonstrates all the characteristics of phytochrome transport in Arabidopsis thaliana. In addition, FHY1 was also actively exported from the nucleus, consistent with its role as a shuttle protein in plants. Therefore, we believe that isolated Acetabularia nuclei may be used as a general tool to study nuclear transport of plant proteins.

Cloning and characterization of the Arabidopsis cyclic phosphodiesterase which hydrolyzes ADP-ribose 1'',2''-cyclic phosphate and nucleoside 2',3'-cyclic phosphates

In eukaryotic cells, pre-tRNAs spliced by a pathway that produces a 3',5'-phosphodiester, 2'-phosphomonoester linkage contain a 2'-phosphate group adjacent to the tRNA anticodon. This 2'-phosphate is transferred to NAD to give adenosine diphosphate (ADP)-ribose 1", 2"-cyclic phosphate (Appr>p), which is subsequently metabolized to ADP-ribose 1"-phosphate (Appr-1"p). The latter reaction is catalyzed by a cyclic phosphodiesterase (CPDase), previously identified in yeast and wheat. In the work presented here, we describe cloning of the Arabidopsis cDNA encoding the 20-kDa CPDase that hydrolyzes Appr>p to Appr-1"p. Properties of the bacterially overexpressed and purified Arabidopsis enzyme are similar to those of wheat CPDase. In addition to their transformation of Appr>p, both enzymes hydrolyze nucleoside 2',3'-cyclic phosphates to nucleoside 2'-phosphates. For the Arabidopsis CPDase, the apparent Km values for Appr>p, A>p, C>p, G>p, and U>p are 1.35, 1.34, 2.38, 16.86, and 17.67 mM, respectively. Southern analysis indicated that CPDase in Arabidopsis is encoded by a single copy gene that is expressed, at different levels, in all Arabidopsis organs that were analyzed. Indirect immunofluorescence, performed with transfected protoplasts, showed that CPDase is localized in the cytoplasm. Based on substrate specificity and products generated, the plant enzyme differs from other known cyclic phosphodiesterases. The Arabidopsis CPDase does not have recognizable structural similarity or motifs in common with proteins deposited in public data bases.

Multiple plant RNA binding proteins identified by PCR: expression of cDNAs encoding RNA binding proteins targeted to chloroplasts in Nicotiana plumbaginifolia

Pre-mRNA processing in eukaryotic cells requires the participation of multiple protein factors and ribonucleoprotein particles. One class of proteins involved in this process are RNA-binding proteins, which contain a domain of ca. 90 amino acids with a characteristic ribonucleoprotein consensus sequence (RNP-CS). A PCR approach that is suitable for the characterization of RNP-CS-type proteins is described. Fifteen different RNA-binding domains were amplified from Nicotiana tabacum (tobacco) using oligonucleotide primers specific for the sequences (K/R)G(F/Y)(G/A)FVX(F/Y) and (L/I/V)(F/Y)(V/I)(G/K)(N/G)L, which are conserved in known RNP-CS proteins. Using the tobacco domains as probes, cDNAs encoding two RNA-binding proteins, each containing two RNP-CS-type domains, were characterized in N. plumbaginifolia. The proteins, designated CP-RBP30 and CP-RBP31, are targeted to chloroplasts as demonstrated by expression of epitope-tagged cDNAs in transfected protoplasts, followed by indirect immunofluorescence. High levels of mRNA for each protein were found in leaves but not in roots, and expression of the CP-RBP31 mRNA was strongly regulated by light. The N. plumbaginifolia proteins described in this work are distinct from chloroplast RNA-binding proteins characterized recently in tobacco and spinach.

Individual selection, culture and manipulation of higher plant cells

Due to the heterogeneity in morphology, physiological and morphogenetical capabilities of higher plant cells in mass culture, the development of methods for individually culturing defined cells seemed to be useful and necessary. Individual cell culture represents a powerful tool for studies on the physiology of different cell types, the analysis of differentiation programs, the genetic manipulation of plant cells and cell-cell interactions. An improved microculture system based on a computer-controlled set-up for the efficient selection, transfer and individual culture of defined higher plant cells until regeneration of whole plants is described. Related experimental approaches for individually manipulating higher plant cells under controlled conditions, such as electrofusion of defined pairs of protoplasts and subprotoplasts, cell reconstruction and intranuclear microinjection of protoplasts and karyoplasts - mainly performed with cells of the crop plant Brassica napus L. - are presented.

Evidence for a circaseptan and a circasemiseptan growth response to light/dark cycle shifts in nucleated and enucleated Acetabularia cells, respectively

Nucleated as well as enucleated Acetabularia mediterranea cells were subjected to 14 different patterns of shifts in a regimen of 12 hr of light alternating with 12 hr of darkness in four 30-day long experiments. With one exception, which might be due to a circannual modulation, these experiments showed that nucleated cells had maximal growth rates when a shift was performed every 7th or 15th day. In enucleated cells, maxima were observed on shift schedules that were about 3-4 days rather than about 7 days apart. The results indicate that in the unicellular green alga Acetabularia a rhythm of about 7 days (circaseptan) exists and that removal of the nucleus results in a circaseptan frequency multiplication.