Fatty acid remodeling: a novel reaction sequence in the biosynthesis of trypanosome glycosyl phosphatidylinositol membrane anchors.

The trypanosome variant surface glycoprotein (VSG) is anchored to the plasma membrane via a glycosyl phosphatidylinositol (GPI). The GPI is synthesized as a precursor, glycolipid A, that is subsequently linked to the VSG polypeptide. The VSG anchor is unusual, compared with anchors in other cell types, in that its fatty acid moieties are exclusively myristic acid. To investigate the mechanism for myristate specificity we used a cell-free system for GPI biosynthesis. One product of this system, glycolipid A', is indistinguishable from glycolipid A except that its fatty acids are more hydrophobic than myristate. Glycolipid A' is converted to glycolipid A through highly specific fatty acid remodeling reactions involving deacylation and subsequent reacylation with myristate. Therefore, myristoylation occurs in the final phase of trypanosome GPI biosynthesis.     

Induced neuroma formation and target muscle perturbation in the giant fiber pathway of the Drosophila temperature-sensitive mutant shibire

Summary The temperature-sensitive mutation shibire (shi) in Drosophila melanogaster is thought to disrupt membrane recycling processes, including endocytotic vesicle pinch-off. This mutation can perturb the development of nerves and muscles of the adult escape response. After exposure to a heat pulse (6 h at 30° C) at 20 h of pupal development, adults have abnormal flight muscles. Wing depressor muscles (DLM) are reduced in number from the normal six to one or two fibers, and are composed of enlarged fibers that appear to represent fiber fusion; large spaces devoid of muscle fibers suggested fiber deletion. The normal five motor axons are present in the peripheral nerve PDMN near the ganglion. However, while some motor axons pass dorsally to the extant fibers, other motor axons lacking end targets pass into an abnormal posterior branch and terminate in a neuroma, i.e., a tangle of axons and glia without muscle target tissue. Hemisynapses are common in axons of the proximal PDMN and within the neuroma, but they are rarely seen in control (no heat pulse) shi or wild-type flies. All surviving muscle fibers are innervated; no muscle tissue exists without innervation. Fibrillar fine structure and neuromuscular synapses appear normal. Fused fibers have dual innervation, suggesting correct and specific matching of target tissue and motor axons. Motor axons lacking target fibers do not innervate erroneous targets but instead terminate in the neuroma. These results suggest developmental constraints and rules, which may contribute to the orderly, stereotyped development in the normal flight system. The nature of the anomalies inducible in the flight motor system in shi flies implies that membrane recycling events at about 20 h of pupal development are critical to the formation of the normal adult nerve-muscle pattern for DLM flight muscles.     

Onset of and recovery from nitrogen stress during reproductive growth of soybean

Photosynthetic rates and allocation of dry matter, nitrogen, and nonstructural carbohydrates were determined during onset of and recovery from a nitrogen stress for reproductive soybean (Glycine max [L.] Merrill cv Ransom) plants. Until the beginning of seed fill, non-nodulated plants were grown in flowing solution culture with 1.0 mM NO3- in a complete nutrient solution. One set of plants then was transferred to minus-nitrogen solution for 24 d of seed fill; a second set was transferred to a minus-nitrogen solution for 14 d followed by return to the complete solution with 1.0 mM NO3- for the remaining 10 d of seed fill; and a third set was continued on the complete solution. Net CO2 exchange rates of individual leaves, which remained nearly constant during seed fill for nonstressed plants, declined at an accelerated rate during onset of nitrogen stress as the specific content of reduced nitrogen in the leaves was decreased by remobilization of nitrogen to support pod growth. The rate of nitrogen remobilization out of leaves initially was relatively greater than the decrease in photosynthetic rate. While rate of pod growth declined in response to the developing nitrogen stress, photosynthetic assimilation of carbon exceeded reproductive demand and nonstructural carbohydrates accumulated within tissues. Following resupply of exogenous NO3-, specific rate of NO3- uptake by roots was enhanced relative to nonstressed plants. While there was little increase in content of reduced nitrogen in leaves, net remobilization of nitrogen out of leaves ceased, and the decline in photosynthetic rate stabilized at about 51% of that for nonstressed plants. This level of photosynthesis, combined with the availability of elevated pools of carbohydrates accumulated during stress, was sufficient to support the increases in both the specific rates of NO3- uptake and the rate of pod growth during recovery.     

Primary Homothallism-relation to Heterothallism in the Regulation of Sexual Morphogenesis in Sistotrema

The wood-rotting basidiomycete, Sistotrema brinkmannii, is an aggregate of biological species possessing several variations—homothallism, bipolar heterothallism and tetrapolar heterothallism—on the genetic regulation of a critical phase of development. Nutritionally forced intra- and interspecific matings provide genetic information about the relatedness of homothallic isolates, the relation of the various species to one another, the genetic basis of homothallism, and its relationship to heterothallism. Most homothallic isolates are interfertile when nutritionally forced. Successful hybridization between species is restricted to particular combinations of homothallic x bipolar isolates. Significant findings of these studies include: (1) documentation of hybridization of biological species in the Homobasidiomycetes, (2) documentation of the relatedness of two naturally occurring, variant systems, homothallism and bipolar heterothallism, that regulate sexual morphogenesis in the higher fungi, (3) evidence for definite, but limited, evolutionary divergence of the polygenic, regulated components of the respective systems, and (4) indication that the genetic basis of homothallism in this system is essentially due to constitutive function and consequently is fundamentally different from presently understood mechanisms in other self-fertile systems.     

MACROCYSTS IN THE LIFE CYCLE OF THE DICTYOSTELIACEAE. I. FORMATION OF THE MACROCYSTS

Macrocysts, a morphogenetic phase that is alternative to sorocarp formation in the life cycle of some cellular slime molds, are known for two genera and five species of the Dictyosteliaceae. In all of these macrocyst formation was found to be strongly affected by four factors: light, temperature, moisture, and the composition of the medium. In general, macrocyst formation was suppressed and sorocarp formation was enhanced by exposure to light, by incubation at temperatures lower than 20 C, by buffering nutrient media with phosphates, and by reducing atmospheric moisture through the use of clay covers on Petri dishes. The extent to which these environmental factors, singly or in combination, inhibited the production of macrocysts varied among the different strains and species.     

HETEROTHALLISM AND SEXUALITY IN ASCOBOLUS STERCORARIUS

Bistis, G. N., and J. R. Raper. (Harvard U., Cambridge, Mass.) Heterothallism and sexuality in Ascobolus stercorarius. Amer. Jour. Bot. 50(9): 880–891. Illus. 1963.—The steps in the sexual development of the heterothallic ascomycete, Ascobolus stercorarius, are: (1) induction of antheridial hyphae and antheridia; (2) induction of ascogonial hyphae and ascogonia; (3) directed growth of the trichogyne; and (4) plasmogamy. Although this sequence occurs in each of the 2 reciprocal combinations (A –antheridial/a-ascogonial and a-antheridial/A-ascogonial), several differences between the 2 combinations have been found. The differences are especially apparent with regard to antheridial induction and the pattern of proliferation of ascogonial hyphae. A study of the specificity of the agents regulating the sexual reactions between the 2 mating-types has confirmed previously described class-specificity at antheridial induction (sexual activation). Experiments utilizing substituted oidia have demonstrated an absence of mating-class specificity in trichogyne attraction and even at plasmogamy. The incipient fruiting bodies which result from illegitimate fusions (a X a and A X A), however, stop growing after 24 hr. This cessation of development suggests the presence of a second block to self-fertility in the sexual process of this species.     

Microcysts of the Cellular Slime Mold Polysphondylium pallidum II. Chemistry of the Microcyst Walls

Methods are described for obtaining large masses of myxamoebae, for inducing these to form microcysts, and for the isolation of the microcyst walls from other cell components. The walls were fractionated into two parts, one alkali-soluble, the other alkali insoluble. The alkali-insoluble fraction is a type of cellulose and constitutes 28% of the microcyst wall by weight. The alkali-soluble fraction contains a glycogenlike material, lipids, and proteins. A possible mechanism of microcyst wall synthesis is discussed.     

A Mushroom-Inducing DNA Sequence Isolated from the Basidiomycete, Schizophyllum Commune

A DNA sequence capable of inducing the de novo development of fruiting bodies (mushrooms) when integrated into the genome of unmated, nonfruiting strains of the Basidiomycete Schizophyllum commune has been isolated and partially characterized. This sequence, designated FRT1, overrides the normal requirement of a mating interaction for fruiting in this organism. It has been shown to integrate stably in different chromosome locations and appears to be trans-acting. It also enhances the normal process of fruiting that occurs after mating. Additional DNA sequences with similarity to FRT1 were detected within the genome of the strain of origin by hybridization of labeled FRT1 DNA to blots of digested genomic DNAs. FRT1 and the genomic sequences similar to it were shown to be genetically linked. Southern hybridization experiments suggested sequence divergence at the FRT1 locus between different strains of S. commune. A testable model for how FRT1 may act as a key element in the pathway for the differentiation of fruiting bodies is presented as a working hypothesis for further investigation.