Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream

1. Epilithic algae grown on elevated or non-elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae. 2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non-elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddisfly larvae when occurring on elevated tiles than on non-elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes. 3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of ’live‘ diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of ’dead‘ (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae. 4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non-elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer-resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer-susceptible taxa [e.g. small, chain-forming Fragilaria (= Staurosirella)]. This result suggests that a trade-off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.     

Pex18p and Pex21p,a Novel Pair of Related Peroxins Essential for Peroxisomal Targeting by the PTS2 Pathway

We have identified ScPex18p and ScPex21p, two novel S. cerevisiae peroxins required for protein targeting via the PTS2 branch of peroxisomal biogenesis. Targeting by this pathway is known to involve the interaction of oligopeptide PTS2 signals with Pex7p, the PTS2 receptor. Pex7p function is conserved between yeasts and humans, with defects in the human protein causing rhizomelic chondrodysplasia punctata (RCDP), a severe, lethal peroxisome biogenesis disorder characterized by aberrant targeting of several PTS2 peroxisomal proteins, but uncertainty remains about the subcellular localization of this receptor. Previously, we have reported that ScPex7p resides predominantly in the peroxisomal matrix, suggesting that it may function as a highly unusual intraorganellar import receptor, and the data presented in this paper identify Pex18p and Pex21p as key components in the targeting of Pex7p to peroxisomes. They each interact specifically with Pex7p both in two-hybrid analyses and in vitro. In cells lacking both Pex18p and Pex21p, Pex7p remains cytosolic and PTS2 targeting is completely abolished. Pex18p and Pex21p are weakly homologous to each other and display partial functional redundancy, indicating that they constitute a two-member peroxin family specifically required for Pex7p and PTS2 targeting.     

Tic20 and Tic22 Are New Components of the Protein Import Apparatus at the Chloroplast Inner Envelope Membrane

Two components of the chloroplast envelope, Tic20 and Tic22, were previously identified as candidates for components of the general protein import machinery by their ability to covalently cross-link to nuclear-encoded preproteins trapped at an intermediate stage in import across the envelope (Kouranov, A., and D.J. Schnell. 1997. J. Cell Biol. 139:1677–1685). We have determined the primary structures of Tic20 and Tic22 and investigated their localization and association within the chloroplast envelope. Tic20 is a 20-kD integral membrane component of the inner envelope membrane. In contrast, Tic22 is a 22-kD protein that is located in the intermembrane space between the outer and inner envelope membranes and is peripherally associated with the outer face of the inner membrane. Tic20, Tic22, and a third inner membrane import component, Tic110, associate with import components of the outer envelope membrane. Preprotein import intermediates quantitatively associate with this outer/inner membrane supercomplex, providing evidence that the complex corresponds to envelope contact sites that mediate direct transport of preproteins from the cytoplasm to the stromal compartment. On the basis of these results, we propose that Tic20 and Tic22 are core components of the protein translocon of the inner envelope membrane of chloroplasts.     

NOTE MOLECULAR ANALYSIS OF THE AHAS GENE OF PORPHYRIDIUM SP. (RHODOPHYTA) AND OF A MUTANT RESISTANT TO SULFOMETURON METHYL

Acetohydroxyacid synthase (AHAS) is the target enzyme of the sulfonylurea herbicides, and here we report the sequence of the gene from wild-type and herbicide-resistant Porphyridium sp. (Rhodophyta). The resistant mutant has a single residue substitution at a position known to confer herbicide resistance in E. coli and in plants. The rhodophyte gene is of cyanobacterial origin and distinct from the nuclear-encoded chlorophyte gene, which may be of mitochondrial origin.     

MOLECULAR DIVERGENCE AND CHARACTERIZATION OF TWO CHLOROPLAST DIVISION GENES,FTSZ1 AND FTSZ2, IN THE UNICELLULAR GREEN ALGA NANNOCHLORIS BACILLARIS (CHLOROPHYTA)1

Two FtsZ paralogues (NbFtsZ1 and NbFtsZ2) were isolated from the unicellular green alga Nannochloris bacillaris Naumann. These sequences encoded proteins of 435 and 439 amino acids with tubulin signature motifs (GGGTG[T/S]G), which are important for GTP binding activity. NbFtsZ1 and NbFtsZ2 had four and three introns, respectively, and two different putative core promoters; a TATA box (TATAAAA) and an initiator element (CCCAGG) were located 40 bp and 80 bp upstream of the coding regions of NbFtsZ1 and NbFtsZ2, respectively. Southern blot hybridization and contour‐clamped homogeneous electric field electrophoresis showed that N. bacillaris contained at least one copy of each gene and that NbFtsZ1 was located on chromosome 5 and NbFtsZ2 on chromosome 3 or 4. Phylogenetically, NbFtsZ1 and NbFtsZ2 belong to the vascular plant protein families FtsZ1 and FtsZ2, respectively. The FtsZ1 proteins do not contain carboxy‐terminal consensus sequences, whereas all FtsZ2 proteins possess the consensus sequence (I/V)PxFL(R/K)(K/R)(K/R). Our study has shown that NbFtsZ2 possesses a similar consensus sequence (VPDFLRRK), whereas NbFtsZ1 does not, further supporting their classification as FtsZ2 and FtsZ1. Escherichia coli ftsZ mutants transformed with cloned NbFtsZ1, and NbFtsZ2 cDNAs were restored for the capacity to divide by binary fission, suggesting that the proteins retained the ability to function in the bacterium. An anti‐NbFtsZ2 antibody specifically recognized a single protein band of approximately 51 kDa on an immunoblot of N. bacillaris cellular proteins. Immunostaining of the algal cells with this antibody produced an intense fluorescent signal as a ring near the middle of the cell, which corresponded to the chloroplast division site.     

The Function of Tocopherols and Tocotrienols in Plants

Referee: Dr. Kozi Asada, Department of Biotechnology, Faculty of Engineering, Fukuyama University, Gakuencho 1, Fukuyama 729-0292, Japan Tocopherols and tocotrienols, which differ only in the degree of saturation of their hydrophobic prenyl side chains, are lipid-soluble molecules that have a number of functions in plants. Synthesized from homogentisic acid and isopentenyl diphosphate in the plastid envelope, tocopherols and tocotrienols are essential to maintain membrane integrity. α-Tocopherol is the major form found in green parts of plants, while tocotrienols are mostly found in seeds. These compounds are antioxidants, thus they protect the plant from oxygen toxicity. Tocopherols and tocotrienols scavenge lipid peroxy radicals, thereby preventing the propagation of lipid peroxidation in membranes, and the ensuing products tocopheroxyl and tocotrienoxyl radicals, respectively, are recycled back to tocopherols and tocotrienols by the concerted action of other antioxidants. Furthermore, tocopherols and tocotrienols protect lipids and other membrane components by physically quenching and reacting chemically with singlet oxygen. The scavenging of singlet oxygen by α-tocopherol in chloroplasts results in the formation of, among other products, α -tocopherol quinone, a known contributor to cyclic electron transport in thylakoid membranes, therefore providing photoprotection for chloroplasts. Moreover, given that α-tocopherol increases membrane rigidity, its concentration, together with that of the other membrane components, might be regulated to afford adequate fluidity for membrane function. Furthermore, α-tocopherol may affect intracellular signaling in plant cells. The effects of this compound in intracellular signaling may be either direct, by interacting with key components of the signaling cascade, or indirect, through the prevention of lipid peroxidation or the scavenging of singlet oxygen. In the latter case, α-tocopherol may regulate the concentration of reactive oxygen species and plant hormones, such as jasmonic acid, within the cell, which control both the growth and development of plants, and also plant response to stress.     

EFFECTS OF LIGHT AND GLYCEROL ON THE ORGANIZATION OF THE PHOTOSYNTHETIC APPARATUS IN THE FACULTATIVE HETEROTROPH PYRENOMONAS SALINA (CRYPTOPHYCEAE)1

The marine cryptophyte Pyrenomonas salina Santore is capable of autotrophic and heterotrophic nutrition. We studied the physiological and ultrastructural changes that accompany the shift between these nutritional modes. The addition of glycerol to batch cultures of P. salina, grown at an irradiance limiting for photoautotrophic growth, increased its growth rate and induced specific biochemical and structural changes in its photosynthetic system. Results from extracted pigment analyses, thin-section electron microscopy, and freeze-fracture electron microscopy indicated that glycerol addition reduced the cell phycoerythrin content, phycoerythrin to chlorophyll a ratio, degree of thylakoid packing, number of thylakoids · cell^?1, and PSII particle size. These properties were reduced to a similar extent in cells grown photoautotrophically under an irradiance saturating for growth. These results are consistent with the hypothesis that enhancement of heterotrophic potential occurs at the expense of light-harvesting ability in glycerol-grown P. salina.     

NUCLEOTIDE SEQUENCE OF THE GENES FOR RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE LARGE SUBUNIT AND RIBOSOMAL PROTEIN S14 FROM A CHLORELLA-LIKE GREEN ALGA1

Two chloroplast genes were sequenced from an exsymbiotic strain of a eukaryotic, Chlorella-like green alga. The genes for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) and the ribosomal protein S14 (rps14) were oriented in the same direction and were separated by 402 bp. The rbcLs of the exsymbiont and a free living Chlorella ellipsoidea were compared with other reported rbcL sequences. The rbcL gene of the exsymbiont is closely related to that of free-living Chlorella ellipsoidea. This is the first published report of an rps 14 gene sequence from an alga.