Essential oil of Ayapana triplinervis from Reunion Island: A good natural source of thymohydroquinone dimethyl ether

Three specimens of Ayapana triplinervis (Vahl) R.M. King & H. Rob from Reunion Island (Indian Ocean) collected at two distant locations (North of the island; samples 1 and 2, South of the island; sample 3), in different growth phases (flowering; samples 1 and 3, vegetative; sample 2) were investigated for their leaf essential oil composition. This study reports the chemical character of this species on the island and investigates the relationship between essential oil composition, developmental stage and geographic location. Analysis by GC–FID and GC–MS enabled us to identify and quantify a total of 39 constituents accounting for 97.1–98.0% of the oils. The three essential oil samples, all obtained by hydrodistillation, showed a high percentage of the aromatic compound thymohydroquinone dimethyl ether (89.9–92.8%). All other minor components remained more or less unchanged both qualitatively and quantitatively with respect to the stage of growth. On the contrary, variations were observed with geographic distribution. The geographical variation of the chemical composition of the volatile oil of A. triplinervis from several sites in the world is also briefly discussed.     

Accumulation of agmatine in chayote (Sechium edule) leaves during development

Arginine, agmatine, putrescine and spermidine were found in the apical parts and leaves of chayote ( Sechium edule Swartz ) at various stages of development. The concentration of agmatine, the immediate decarboxylation product of L-arginine, increased considerably in young leaves at the first emergence of floral buds. Young leaves always had a relatively higher content of agmatine than older ones. There was a decrease in the concentration of agmatine from the apical part to the basal leaves. Agmatine was the predominant amine in young leaves at every stage of development (50–90% of the whole amine pool). It was also predominant in mature leaves when the floral buds appeared (70% of the total amine content). An accumulation of agmatine could not be found in other Cucurbitaceae species.     

Colicin import and pore formation: a system for studying protein transport across membranes?

Pore-forming colicins are a family of protein toxins (M_r40–70kDa) produced by Escherichia coli and related bacteria. They are bactericidal by virtue of their ability to form ion channels in the inner membrane of target cells. They provide a useful means of studying questions such as toxin action, polypeptide translocation across and into membranes, voltage-gated channels and receptor function. These colicins bind to a receptor in the outer membrane before being translocated across the cell envelope with the aid of helper proteins that belong to nutrient-uptake systems and the so-called‘Tol’proteins, the function of which has not yet been properly defined. A distinct domain appears to be associated with each of three steps (receptor binding, translocation and formation of voltage-gated channels). The Tol-dependent uptake pathway is described here. The structures and interactions of TolA, B, Q and R have by now been quite clearly defined. Transmembrane α-helix interactions are required for the functional assembly of the E. coli Tol complex, which is preferentially located at contact sites between the inner and outer membranes. The number of colicin translocation sites is about 1000 per cell. The role and the involvement of the OmpF porin (with colicins A and N) have been described in a recent study on the structural and functional interactions of a colicin-resistant mutant of OmpF. The X-ray crystal structure of the channel-forming fragment of colicin A and that of the entire colicin la have provided the basis for biophysical and site-directed muta-genesis studies. Thanks to this powerful combination, it has been established that the interaction with the receptor in the outer membrane leads to a very substantial conformational change, as a result of which the N-terminal domains of colicins interact with the lumen of the OmpF pore and then with the C-terminal domain of TolA. A molten globular conformation of colicins probably constitutes the intermediate translocation/insertion competent state. Once the pore has formed, the polypeptide chain spans the whole cell envelope. Three distinct steps occur in the last stage of the process: (i) fast binding of the C-terminal domain to the outer face of the cytoplasmic membrane; (ii) a slow insertion of the polypeptide chain into the outer face of the inner membrane in the absence of Δψ and (iii) a profound reorganization of the helix association, triggered by the transmembrane potential and resulting in the formation of the colicin channel.     

Identification of mitochondrial proteins on two-dimensional electrophoresis gels of extracts of adult Drosophila melanogaster

Several hundred proteins have been resolved on two-dimensional gels of extracts of [³⁵S]methionine-labeled adult Drosophila melanogaster. 27 of these polypeptides disappear from the gel pattern after feeding the K⁺ ionophore nonactin. These proteins have been identified as mitochondrial, since the two-dimensional gel pattern of extracts of isolated mitochondria correlates well with the pattern of the proteins missing from that of nonactin-treated flies. Nine new proteins also appear on the two-dimensional gels of the extracts from the nonactin-treated flies. Apparently, these nine proteins are precursors of the mature mitochondrial forms. These particular data support the concept that processing of many of the cytoplasmically synthesized mitochondrial proteins requires a specific membrane potential, and that some of these proteins are modified intramitochondrially. However, using [³⁵S]methionine incorporation techniques, not all labeled polypeptides disappear from mitochondria during such treatment. Feeding similarly radiolabeled flies with chloramphenicol, an inhibitor of mitochondrial protein synthesis, results in the disappearance of only one protein from the gel pattern with the concurrent appearance of a ‘new’ high-molecular-weight polypeptide. Collectively, these data show that a specific group of [³⁵S]methionine-labeled mitochondrial proteins can be identified by selective inhibition of mitochondrial function in whole cell protein maps of adult D. melanogaster.     

A 126 bp fragment of a plant histone gene promoter confers preferential expression in meristems of transgenic Arabidopsis

The tissue-specific pattern of expression directed by the H4A748 Arabidopsis histone promoter was investigated by analysis of beta-glucuronidase (GUS) activity in transgenic Arabidopsis containing H4A748-GUS gene fusions. As determined by fluorimetric and histochemical tests, the H4A748 promoter directs preferential expression in meristems of young seedlings and adult plants. The low activity found in nonproliferating tissues may relate to basal constitutive expression of the histone promoter and/or to endoreduplication occurring in some tissues. The endogenous histone mRNA levels parallel the GUS activity found in different tissues. Analysis of the regulatory properties of 5' deleted promoters showed that multiple positive elements exist between -900 and -219 and that the proximal region of the promoter to -219 is sufficient to establish the full tissue-specific pattern of expression. Further deletion to -93 nearly abolished the promoter activity thus suggesting that the 126 bp fragment located between -219 and -93 contains the elements responsible for the specific expression pattern. The presence of several remarkable sequences within this fragment is discussed.