Low temperature delays development of photosystem II activity in winter rye leaves

The ability of leaves to acclimate photosynthetically to low temperature was examined during leaf development in winter rye plants ( Secale cereale L. cv. Puma) grown at 20°C or at 6°C. All leaves grown at 6°C exhibit increased chlorophyll (Chl) levels per leaf area, higher rates of uncoupled, light-saturated photosystem I (PSI) electron transport, and slower increases in photosystem II (PSII) electron transport capacity, when compared with 20°C leaves. The stoiehiometry of PSI and PSII was estimated for each leaf age class by quantifying Chl in elcctrophorctic separations of Chl-protein complexes. The ratio of PSII/PSI electron transport in 20°C leaves is highly correlated with the ratio of core Chl a -proteins associated with PSII (CPa) to those associated with PSI (CP1). In contrast, PSII/PSI electron transport in 6°C leaves is not as well correlated with CPa/CP1 and is related, in part, to the amount and organization of light-harvesting Chl a/b -proteins associated with PSII. CPa/CP1 increases slowly in 6°C leaves, although the ratio of CPa/CP1 in mature 20°C and 6°C leaves is not different. The results suggest that increased PSI activity at low temperature is not related to an increase in the relative proportion of PSI and may reflect, instead, a regulatory change. Photosynthetic acclimation to low environmental temperature involves increased PSI activity in mature leaves shifted to 6°C. In leaves grown entirely at 6°C, however, acclimation includes both increased PSI activity and modifications in the rate of accumlation of PSII and in the organization of LHCII.     

Low Temperature-Induced Decrease in trans-Delta-Hexadecenoic Acid Content Is Correlated with Freezing Tolerance in Cereals

The effect of growth at 5°C on the trans-Δ³-hexadecenoic acid content of phosphatidyl(d)glycerol was examined in a total of eight cultivars of rye (Secale cereale L.) and what (Triticum aestivum L.) of varying freezing tolerance. In these monocots, low temperature growth caused decreases in the trans-Δ³-hexadecenoic acid content of between 0 and 74% with concomitant increases in the palmitic acid content of phosphatidyl(d)glycerol. These trends were observed for whole leaf extracts as well as isolated thylakoids. The low growth temperature-induced decrease in the trans-Δ³-hexadecenoic acid content was shown to be a linear function (r² = 0.954) of freezing tolerance in these cultivars. Of the six cold tolerant dicotyledonous species examined, only Brassica and Arabidopsis thaliana L. cv Columbia exhibited a 42% and 65% decrease, respectively, in trans-Δ³-hexadecenoic acid content. Thus, the relationship between the change in trans-Δ³-hexadecenoic acid content of phosphatidyl(d)glycerol and freezing tolerance cannot be considered a general one for all cold tolerant plant species. However, species which exhibited a low growth temperature-induced decrease in trans-Δ³-hexadecenoic acid also exhibited a concomitant shift in the in vitro organization of the light harvesting complex II from a predominantly oligomeric form to the monomeric form. We conclude that the proposed role of phosphatidyl(d)glycerol in modulating the organization of light harvesting complex II as a function of growth temperature manifests itself to varying degrees in different plant species. A possible physiological role for this phenomenon with respect to low temperature acclimation and freezing tolerance in cereals is discussed.     

Antisense genes in plants: an overview

Plants are the first multicellular higher eukaryotic organisms in which artificial antisense genes have been shown to down-regulate target gene expression. Manipulations with an antisense gene can serve as a tool to study the effect of a particular plant gene inactivation, the interaction of gene products whose genes are coordinately expressed, or the functional analysis of cryptic genes. Transgenic plants harbouring an antisense gene already gave rise to patentable new characteristics, showing that the technique has great scientific and economic value.     

Overwintering Periwinkle (Vinca minor L.) Exhibits Increased Photosystem I Activity

The effects of natural, overwintering conditions on photosystem I and photosystem II activity were examined in isolated thylakoids of periwinkle (Vinca minor L.), an endemic, cold-tolerant, herbaceous evergreen. DCMU-Insensitive photosystem I activity (ascorbate/dichlorophenolindophenol → methylviologen) exhibited a twofold increase in light-saturated rates upon exposure to low temperature and freezing stress with no effect on the apparent quantum yield of this reaction. DCMU-Sensitive photosystem II activity (H₂O → dichlorlophenolindophenol) exhibited only minor fluctuations in light-saturated rates but a 50% decrease in the apparent quantum yield of this reaction upon exposure to overwintering conditions. This was correlated with a decrease in the 77°K fluorescence emission at 694 nanometers. These functional changes occurred with no detectable changes in the relative chlorophyll contents of the chlorophyll-protein complexes or the chlorophyll-thylakoid protein. The chlorophyll a/b varied less than 10% during any single growth year. Analyses of total leaf extracts indicated that all lipid classes exhibited increased levels of linoleic and linolenic acid. Neither the trans-Δ³-hexadecenoic acid level nor the ratio of oligomeric:monomeric light harvesting of photosystem II was affected by exposure to winter stress. The content of the major chloroplast lipids monogalactosyldiacylglycerol, digalactosyldiacylglycerol, phosphatidyl-diacyl-glycerol, and sulfoquinovosyldiacylglycerol exhibited minor fluctuations, whereas phosphatidylcholine and phosphatidylethanolamine content doubled on a mole percent or chlorophyll basis. We conclude that the previously reported increase in photosystem I activity during controlled, low temperature growth is observed during exposure to natural overwintering conditions. This appears to occur with minimal changes in the structure and composition of the photosynthetic apparatus of periwinkle.     

An additional long-range interaction in human U1 snRNA.

We present evidence for the existence of an additional long-range interaction in vertebrate U1 snRNAs. By submitting human U1 snRNP, HeLa nuclear extracts, authentic human or X. laevis in vitro transcribed U1 snRNAs to RNase V1, a nuclease specific for double-stranded regions, cleavages occurred in the sequence psi psi ACC (positions 5-9) residing in the 5' terminal region of the RNA. The RNase V1 sensitive region is insensitive to single-stranded probes, something unexpected knowing that it was considered single-stranded in order to base-pair to pre-mRNA 5' splice site. We have identified the sequence GGUAG (positions 132-136) as the only possible 3' partner. Mutants, either abolishing or restoring the interaction between the partners, coupled to an RNase V1 assay, served to substantiate this base-pairing model. The presence of this additional helix, even detected in nuclear extracts under in vitro splicing conditions, implies that a conformational change must occur to release a free U1 snRNA 5' end.     

A protein binds the selenocysteine insertion element in the 3'-UTR of mammalian selenoprotein mRNAs.

Several gene products are involved in co-translational insertion of selenocysteine by the tRNA(Sec). In addition, a stem-loop structure in the mRNAs coding for selenoproteins is essential to mediate the selection of the proper selenocysteine UGA codon. Interestingly, in eukaryotic selenoprotein mRNAs, this stem-loop structure, the selenocysteine insertion sequence (SECIS) element, resides in the 3'-untranslated region, far downstream of the UGA codon. In view of unravelling the underlying complex mechanism, we have attempted to detect RNA-binding proteins with specificity for the SECIS element. Using mobility shift assays, we could show that a protein, present in different types of mammalian cell extracts, possesses the capacity of binding the SECIS element of the selenoprotein glutathione peroxidase (GPx) mRNA. We have termed this protein SBP, for Secis Binding Protein. Competition experiments attested that the binding is highly specific and UV cross-linking indicated that the protein has an apparent molecular weight in the range of 60-65 kDa. Finally, some data suggest that the SECIS elements in the mRNAs of GPx and another selenoprotein, type I iodothyronine 5' deiodinase, recognize the same SBP protein. This constitutes the first report of the existence of a 3' UTR binding protein possibly involved in the eukaryotic selenocysteine insertion mechanism.