Phylogenetic evidence for horizontal transmission of group I introns in the nuclear ribosomal DNA of mushroom-forming fungi

Group I introns were discovered inserted at the same position in the nuclear small-subunit ribosomal DNA (nuc-ssu-rDNA) in several species of homobasidiomycetes (mushroom-forming fungi). Based on conserved intron sequences, a pair of intron-specific primers was designed for PCR amplification and sequencing of intron-containing rDNA repeats. Using the intron-specific primers together with flanking rDNA primers, a PCR assay was conducted to determine presence or absence of introns in 39 species of homobasidiomycetes. Introns were confined to the genera Panellus, Clavicorona, and Lentinellus. Phylogenetic analyses of nuc-ssu-rDNA and mitochondrial ssu-rDNA sequences suggest that Clavicorona and Lentinellus are closely related, but that Panellus is not closely related to these. The simplest explanation for the distribution of the introns is that they have been twice independently gained via horizontal transmission, once on the lineage leading to Panellus, and once on the lineage leading to Lentinellus and Clavicorona. BLAST searches using the introns from Panellus and Lentinellus as query sequences retrieved 16 other similar group I introns of nuc-ssu-rDNA and nuclear large-subunit rDNA (nuc-lsu-rDNA) from fungal and green algal hosts. Phylogenetic analyses of intron sequences suggest that the mushroom introns are monophyletic, and are nested within a clade that contains four other introns that insert at the same position as the mushroom introns, two from different groups of fungi and two from green algae. The distribution of host lineages and insertion sites among the introns suggests that horizontal and vertical transmission, homing, and transposition have been factors in intron evolution. As distinctive, heritable features of nuclear rDNAs in certain lineages, group I introns have promise as phylogenetic markers. Nevertheless, the possibility of horizontal transmission and homing also suggest that their use poses certain pitfalls.      

Characterization of a Polyphosphoinositide Phospholipase C from the Plasma Membrane of Avena sativa

A phosphoinositide-specific phospholipase C activity was identified in oat root (Avena sativa, cv Victory) plasma membranes purified by separation in an aqueous two-phase polymer system. The enzyme is highly active toward inositol phospholipids but only minimally active toward phosphatidylethanolamine and phosphatidylcholine. Activity approaches maximal levels at 200 micromolar phosphatidylinositol 4-phosphate (PIP) and is highly dependent on calcium; it is inhibited by 1 millimolar EGTA and is activated by calcium with an apparent activation constant of 2 micromolar. At 10 micromolar calcium and 200 micromolar inositol phospholipid, the enzyme is specific for phosphatidylinositol 4,5-bisphosphate (PIP₂) and PIP, which are hydrolyzed at 10 and 4 times, respectively, the rate of phosphatidylinositol (PI) hydrolysis. The principle water soluble products of hydrolysis, as determined by high performance liquid chromatography, are inositol 1,4,5-trisphosphate from PIP₂, inositol 1,4-bisphosphate from PIP, and inositol phosphate from PI.     

The sequence of a sea urchin muscle actin gene suggests a gene conversion with a cytoskeletal actin gene

Summary We report the nucleotide sequence of the single muscle actin gene of the sea urchinStrongylocentrotus purpuratus. Comparison of the protein-coding sequence of this muscle actin gene (pSpG28) with that of two linked sea urchin cytoskeletal actin genes (pSpG17 and CyIIa) reveals a region of exceptional sequence conservation from codon 61 through codon 120. Furthermore, when silent nucleotide changes are compared, the conservation of this region is still evident (7.9% silent site differences in the conserved region vs 43.3% silent site differences in the rest of the gene when pSpG28 and CyIIa are compared), indicating that the conservation is not due to particularly stringent selection on the portion of the protein encoded by this region of the genes. These observations suggest that a gene conversion has occurred between the muscle actin gene and a cytoskeletal actin gene recently in the evolution of the sea urchin genome. Gene conversion between nonallelic actin genes may thus play a role in maintaining the homogeneity of this highly conserved gene family.     

The possible role of "sibling neurite bias" in the coordination of neurite extension, branching, and survival

In this review we consider a novel mechanism, "sibling neurite bias," which may explain aspects of the coordination of elongation, branching, and resorption among different neurites growing from the same neuronal cell body. In this model, growing neurites which incorporate structural precursors at higher rates would deplete the cellular pool of precursors available to their "sibling" neurites; neurites would compete for survival, but in addition they would bias each other's behavior during active growth. Evidence is reviewed that "sibling neurite bias" may contribute to the establishment and stabilization of specific neural connections. Specific examples examined include the loss of polyinnervation at the developing neuromuscular junction, contextual mapping in the retino-tectal system, and selective neurite growth patterns and synaptic connections in nerve tissue culture model systems.     

Structure determination of a new fluorescent tricyclic nucleoside from archaebacterial tRNA.

A highly fluorescent nucleoside was detected in enzymatic digests of the extremely thermophilic archaebacterium Sulfolobus solfataricus by combined liquid chromatography-mass spectrometry (LC/MS). Following isolation, the structure was determined primarily by mass spectrometry, to be 3-(beta-D-ribofuranosyl)-4,9-dihydro-4,6,7-trimethyl-9-oxoimidazo[ 1, 2-a]purine (mimG), a new derivative of the Y (wye) nucleoside. The structural assignment was verified by comparison of the base released by acid hydrolysis with the corresponding synthetic base, using mass spectrometry, chromatography, and UV absorption and fluorescence properties. Nucleoside mimG was also detected by LC/MS in hydrolysates of the thermophiles Thermoproteus neutrophilus and Pyrodictium occultum. These results constitute the first finding of a member of the hypermodified Y family of nucleosides in archaebacteria.     

Rainfall-driven sex-ratio genes in African buffalo suggested by correlations between Y-chromosomal haplotype frequencies and foetal sex ratio

Background  

The Y-chromosomal diversity in the African buffalo (Syncerus caffer) population of Kruger National Park (KNP) is characterized by rainfall-driven haplotype frequency shifts between year cohorts. Stable Y-chromosomal polymorphism is difficult to reconcile with haplotype frequency variations without assuming frequency-dependent selection or specific interactions in the population dynamics of X- and Y-chromosomal genes, since otherwise the fittest haplotype would inevitably sweep to fixation. Stable Y-chromosomal polymorphism due one of these factors only seems possible when there are Y-chromosomal distorters of an equal sex ratio, which act by negatively affecting X-gametes, or Y-chromosomal suppressors of a female-biased sex ratio. These sex-ratio (SR) genes modify (suppress) gamete transmission in their own favour at a fitness cost, allowing for stable polymorphism.     

Biosynthesis of glycerolipids by hepatoma and liver microsomes. I. Fatty acyl-CoA ligase and acyl-CoA:sn-glycerol-3-phosphate acyltransferase

Highly purified plasma membranes of calf thymocytes were fractionated by means of affinity chromatography on ouabain-Sepharose. By the method used two subfractions were obtained, one eluting freely from the affinity gel (MF1oua) and a second specifically retained by matrix-bound ouabain (MF2oua), with a total recovery of 95 per cent. Fractionation required the binding of matrix-bound ouabain to its plasma membrane receptor, i.e. (Na+ + K+)-ATPase. Increasing the temperature and binding time did not significantly alter the fractionation of plasma membranes into the two subfractions. Both plasma membrane subfractions separated by ouabain-Sepharose were of plasma membrane origin, as revealed by the identical specific activities of several membrane bound enzymes, gamma-glutamyl transpeptidase, alkaline phosphatase and Mg2+-ATPase in unseparated plasma membranes and in both subfractions, and by the identical amounts of the cytoskeletal protein actin in unseparated plasma membranes and subfractions. The plasma membrane subfractions MF1oua and MF2oua showed different structural and functional properties. In SDS-polyacrylamide gel electrophoresis polypeptides of 170, 150, 110, 94, 39, and 30 kDa were several-fold enriched in the adherent fraction, MF2oua. The phospholipid fatty acid composition of the plasma membrane subfractions proved to be different, as well. MF2oua contained significantly higher amounts of saturated fatty acids as compared to MF1oua. The specific activities of (Na+ + K+)-ATPase, Ca2+-ATPase and lysolecithin acyltransferase were highly enriched in the adherent fraction MF2oua, as compared to MF1oua. The data suggest that by the means of affinity chromatography on ouabain-Sepharose plasma membrane domains of the lymphocyte plasma membrane can be isolated, most probably implicated in the initiation of lymphocyte activation.