Occurrence, characterization and development of two different types of microbodies in the nematophagous fungus Arthrobotrys oligospora

Abstract The occurrence of microbodies in different cells of the nematophagous fungus Arthrobotrys oligospora has been investigated. In the predacious phase this organism forms complex 3-dimensional network traps. Mature trap cells generally were crowded with "special" microbodies which possessed an electron dense matrix and were surrounded by a membrane of approx. 9 nm. These organelles developed during the early stages of trap formation and were derived from specialized regions of the endoplasmic reticulum. Cytochemical staining experiments revealed that the electron-dense microbodies contained catalase and d -amino acid oxidase and thus must be considered peroxisomal in nature. Electron-dense bodies were absent in normal vegetative cells of the fungus. These cells contained "normal" microbodies which developed from each other by the separation of small organelles from mature ones. As in yeasts, the metabolic function of these latter organelles was dependent upon environmental conditions.     

Functional Analysis of the N-Terminal Prepeptides of Watermelon Mitochondrial and Glyoxysomal Malate Dehydrogenases*

Mitochondrial and glyoxysomal malate dehydrogenase (mMDH; gMDH; L-malate: NAD⁺ oxidoreductase; EC 1.1.1.37) of watermelon (Citrullus vulgaris) cotyledons are synthesized with N-terminal cleavable presequences which are shown to specify sorting of the two proteins. The two presequences differ in length (27 or 37 amino acids) and primary structure. Precursor proteins of the two isoenzymes with site-directed mutations in their presequences and hybrid precursor proteins with reciprocally exchanged presequences were analyzed for proper import using two approaches, namely in vitro using isolated watermelon organelles or in vivo after synthesis in the heterologous host, Hansenula polymorpha. The mitochondrial presequence is essential and sufficient to target the mature glyoxysomal isoenzyme into mitochondria (Gietl et al., 1994). As to the function of the mitochondrial presequence a substitution of ^?3R (considered important for one step precursor cleavage in yeast and mammals) with ^?3L permitted import into mitochondria but cleavage of the transit peptide and conversion into active mature enzyme was impeded. Substitution of ^?13R^?12S (in a sequence reminiscent of the octapeptide motif serving as a substrate for the mammalian and yeast intermediate peptidase) into ^?13L¹²F permitted mitochondrial import and processing like the wild type transit peptide. Purified rat mitochondrial processing protease, which can effect single step cleavage of mitochondrial protein precursors, cleaves in vitro translated watermelon mMDH precursor into its mature form. The glyoxysomal presequence is essential and sufficient to target the mature mitochondrial isoenzyme into peroxisomes of Hansenula polymorpha, but these peroxisomes lack a processing enzyme to cleave the presequence (Gietl et al., 1994). We here show that isolated watermelon organelles also import the hybrid proteins in vitro and process the glyoxysomal presequence. Site directed mutations within the conserved RI-X₅-HL-motif impede efficiency of import and cleavage by watermelon organelles.     

Electron microscopy of septa in ascomycetous yeasts

Electron micrographs of septa in ascomycetous, filamentous yeasts showed three types of structure, namely, dolipores, plasmodesmata, and a central connection indicated as a closure line. The taxonomic use of these and some other morphological features is surveyed.