The effects of date of planting on field establishment of serotinous cape Proteaceae

Season of fire have marked effects on the germination and establishment of serotinous shrubs of the family Proteaceae in fynbos vegetation. To investigate reasons for this, we simulated the effects of different fire seasons by planting seeds into cleared fynbos and then followed their progress. Four species of Proteaceae were planted monthly at four sites over two and a half years. Exclosures were used to exclude rodent seed predators. Germination was confined largely to the three winter months (June–Aug.). Seeds planted from January–June had higher germination than those planted in the second half of the year. Higher levels of regeneration noted after fires in the first half of the year, were previously hypothesised to be results of predation. However, we obtained similar results despite the exclusion of seed predators. Monthly minimum temperature was strongly correlated with germination percentage but monthly rainfall was not. Loss of seed viability may be important, in determining post-fire seedling densities. Differential seedling mortality of earlier and late germinants appears to be unimportant in determining establishment levels. Our results nevertheless support the current practice of restricting management fires in fynbos to the summer-autumn period.     

Characterization of the fusogenic properties of Sendai virus: kinetics of fusion with erythrocyte membranes

A novel fluorescence assay [Hoekstra, D., De Boer, T., Klappe, K., & Wilschut, J. (1984) Biochemistry 23, 5675-5681] has been used to characterize the fusogenic properties of Sendai virus, using erythrocyte ghosts and liposomes as target membranes. This assay involves the incorporation of the "fusion-reporting" probe in the viral membrane, allowing continuous monitoring of the fusion process in a very sensitive manner. Fusion was inhibited upon pretreatment of Sendai virus with trypsin. Low concentrations of the reducing agent dithiothreitol (1 mM) almost completely abolished viral fusion activity, whereas virus binding was reduced by ca. 50%, indicating that the fusogenic properties of Sendai virus are strongly dependent on the integrity of intramolecular disulfide bonds in the fusion (F) protein. Pretreatment of erythrocyte ghosts with nonlabeled Sendai virus inhibited subsequent fusion of fluorophore-labeled virus irrespective of the removal of nonbound virus, thus suggesting that the initial binding of the virus to the target membrane is largely irreversible. As a function of pH, Sendai virus displayed optimal fusion activity around pH 7.5-8.0. Preincubation of the virus at suboptimal pH values resulted in an irreversible diminishment of its fusion capacity. Since virus binding was not affected by the pH, the results are consistent with a pH-induced irreversible conformational change in the molecular structure of the F protein, occurring under mild acidic and alkaline conditions. In contrast to virus binding, fusion appeared to be strongly dependent on temperature, increasing ca. 25-fold when the temperature was raised from 23 to 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Type 1C fimbriae of a uropathogenic Escherichia coli strain: cloning and characterization of the genes involved in the expression of the 1C antigen and nucleotide sequence of the subunit gene

The genes responsible for expression of type 1C fimbriae have been cloned from the uropathogenic Escherichia coli strain AD110 in the plasmid vector pACYC184. Analysis of deletion mutants from these plasmids showed that a 7-kb DNA fragment was required for biosynthesis of 1C fimbriae. Further analysis of this DNA fragment showed that four genes are present encoding proteins of 16, 18.5, 21 and 89 kDal. A DNA fragment encoding the 16-kDal fimbrial subunit has been cloned. The nucleotide sequence of the structural gene and of the C- and N-terminal flanking regions was determined. The structural gene codes for a polypeptide of 181 amino acids, including a 24-residue N-terminal signal sequence. The nucleotide sequence and the deduced amino acid sequence of the 1C subunit gene were compared with the sequences of the fimA gene, encoding the type 1 fimbrial subunit of E. coli K-12. The data show absolute homology at the N- and C-termini; there is less, but significant homology in the region between the N- and C-termini. Comparison of the amino acid compositions of the 1C and FimA subunit proteins with those of the F72 and PapA proteins (subunits for P-fimbriae) revealed that homology between these two sets of fimbrial subunits is also maximal at the N- and C-termini.     

Molecular organisation of the genes involved in the production of F7(2) fimbriae, causing mannose-resistant haemagglutination, of a uropathogenic Escherichia coli 06:K2:H1:F7 strain

Summary The genes responsible for the formation of the F7₂ fimbriae of the uropathogenic E. coli strain AD110 (O6:K2:H1:F7) have been cloned on the recombinant plasmid pPIL110-35 (Van Die et al. 1983). The F7₂ fimbriae, like the F7₁ fimbriae of AD110, are responsible for mannose resistant haemagglutination (MRHA).The molecular organisation of the genes of pPIL110-35 involved in the expression of MRHA was studied by: (a) analysis of transposon and Tn5 insertion mutants. Mutations that cause an MRHA-deficient phenotype were located in discrete groups within an 11.5 kb restriction fragment of pPIL110-35, separated by insertion mutations that do not inactivate MRHA. (b) complementation experiments. Restriction fragments of pPIL110-35 subcloned in the vector pBR322 were tested for their ability to complement transposon insertion mutations in the corresponding regions of pPIL110-35. Five complementation groups were distinguished.Five genes (designated A-E) involved in the expression of MRHA can be distinguished by these results. The products of these genes were analysed in minicells. The results indicate that gene B codes for a 75 K dalton protein, gene C for a 23 K dalton protein and gene E for a 36 K dalton protein. No product of gene D was observed. Gene A probably codes for the 17 K dalton subunit polypeptide of the F7₂ fimbriae, as will be discussed.     

Ultrastructural localization of zinc in rat exocrine pancreas by autoradiography

Summary ⁶⁵Zn was infused at a constant rate for 10 days into a rat. Glutaraldehyde fixed, Epon-araldite embedded ultrathin sections of pancreatic tissue were coated with Ilford L4 emulsion and at 211 days exposure were developed. Silver grains were found over the zymogen granules and over the rough endoplasmic reticulum of exocrine cells. Islet tissue was not observed in these studies. The failure of other zinc localization methods to demonstrate zinc in acinar tissue is discussed as are some of the pitfalls of the autoradiographic method and suggestions for future improvement.Published with the approval of the Director of the Wisconsin Agricultural Experiment Station, Madison. Supported in part by USPHS Research Grant AM-05606 from the Nat. Institute of Arthritis and Metabolic Diseases.Supported by an NIH post-doctoral fellowship.     

The evolution of male-female dimorphism: Older than sex?

This contribution considers the evolution of a dimorphism with respect to cell fusion characteristics in a population of primitive cells. These cells reproduce exclusively asexually. The evolution towards asymmetric fusion behaviour of cells is driven by selection promoting horizontal transfer of an endosymbiontic replicator. It is concluded that evolution of asymmetric cell fusion in this scenario is more likely than evolution of sexual differentiation in a sexually reproducing population. Pre-existing dimorphism with respect to cell fusion may thus have been the basis for the establishment of sexual differentiation at the level of gamete fusion, and this in turn is fundamental to the evolution of two different sexes, male and female.     

Fusion of enveloped viruses with cells and liposomes

The fusion of viruses with cells and liposomes is reviewed with focus on the analysis of the final extents and kinetics of fusion.Influenza virus andSendai virus exhibit 100% of fusion capacity with cells at pH 5 and pH 7.5, respectively. On the other hand, there may be in certain cases, a limit on the number of virions that can fuse with a single cell, that is significantly below the limit on binding. It still remains to be resolved whether this limit reflects a limited number of possible fusion sites, or a saturation limit on the amount of viral glycoproteins that can be incorporated in the cellular membrane, like the case of virus fusion with pure phospholipid vesicles, in which the fusion products were shown to consist of a single virus and several liposomes. Both viruses demonstrate incomplete fusion activity towards liposomes of a variety of compositions. In the case ofSendai virus, fusion inactive virions bind essentially irreversibly to liposomes. Yet, preliminary results revealed that such bound, unfused virions can be released by sucrose gradient centrifugation. The separated unfused virions subsequently fuse when incubated with a “fresh” batch of liposomes. We conclude, therefore, that the fraction of initially bound unfused virions does not consist of dective particles, but rather of particles bound to liposomes via “inactive” sites. Details of the low pH inactivation of fusion capacity ofinfluenza virus towards cells and liposomes are presented. This inactivation is caused by protonation and exposure of the hydrophobic segment of HA₂, and affects primarily the fusion rate constants. Some degree of inactivation also occurs when virions are bound to cellular membranes.     

A novel family of linear plasmids with homology to plasmid pAL2-1 of Podospora anserina

Three recently isolated wild-type strains of the ascomycete Podospora anserina were analyzed for the presence of linear mitochondrial plasmids. In one of these strains, designated Wa6, at least 12 distinct plasmid-like elements were identified. From molecular analyses a minimum number of 78 individual linear molecules with proteins bound to their 5′ ends was estimated. In addition, the different members of this family of typical linear plasmids were shown to possess a common central region and terminal sequences which differ from one plasmid to another due to the presence of different numbers of a 2.4 kb sequence module. Finally, the pWa6 plasmids share a high degree of sequence similarity with pAL2-1, a linear plasmid previously identified in mitochondria of a long-lived mutant of P.anserina. A mechanism is proposed which explains the generation of these distinct, closely related extrachromosomal genetic traits.     

Characterization of two protein kinases from Schizosaccharomyces pombe involved in the regulation of DNA repair.

We have identified two novel genes designated hhp1+ and hhp2+ in the fission yeast Schizosaccharomyces pombe. The hhp1+ and hhp2+ genes encode two closely related protein kinases that share significant sequence identities with Hrr25p from Saccharomyces cerevisiae. Characterization of strains harboring single and double mutations in the hhp+ genes reveals DNA repair defects in these cells. Schizosaccharomyces pombe strains lacking either or both Hhp activities reveal differences in their ability to withstand DNA lesions caused by either methyl methanesulfonate (MMS) or gamma-rays which correlate with their ability to repair DNA strand breaks caused by these agents. We suggest that Hhp1 and Hhp2 are involved in the regulation of distinct and overlapping DNA repair pathways in S. pombe.