A Multi-level Blocking Distinct Degree Factorization Algorithm

We give a new algorithm for performing the distinct-degree factorization of a polynomial P(x) over GF(2), using a multi-level blocking strategy. The coarsest level of blocking replaces GCD computations by multiplications, as suggested by Pollard (1975), von zur Gathen and Shoup (1992), and others. The novelty of our approach is that a finer level of blocking replaces multiplications by squarings, which speeds up the computation in GF(2)[x]/P(x) of certain interval polynomials when P(x) is sparse. As an application we give a fast algorithm to search for all irreducible trinomials x^r + x^s + 1 of degree r over GF(2), while producing a certificate that can be checked in less time than the full search. Naive algorithms cost O(r^2) per trinomial, thus O(r^3) to search over all trinomials of given degree r. Under a plausible assumption about the distribution of factors of trinomials, the new algorithm has complexity O(r^2 (log r)^{3/2}(log log r)^{1/2}) for the search over all trinomials of degree r. Our implementation achieves a speedup of greater than a factor of 560 over the naive algorithm in the case r = 24036583 (a Mersenne exponent). Using our program, we have found two new primitive trinomials of degree 24036583 over GF(2) (the previous record degree was 6972593)

Import of honeybee prepromelittin into the endoplasmic reticulum

Honeybee prepromelittin is correctly processed and imported by dog pancreas microsomes. Insertion of prepromelittin into microsomal membranes, as assayed by signal sequence removal, does not depend on signal recognition particle (SRP) and docking protein. We addressed the question as to how prepromelittin bypasses the SRP/docking protein system. Hybrid proteins between prepromelittin, or carboxy-terminally truncated derivatives, and the cytoplasmic protein dihydrofolate reductase from mouse were constructed. These hybrid proteins were analysed for membrane insertion and sequestration into microsomes. The results suggest the following: (i) The signal sequence of prepromelittin is capable of interacting with the SRP/docking protein system, but this interaction is not mandatory for membrane insertion; this is related to the small size of prepromelittin. (ii) In prepromelittin a cluster of negatively charged amino acids must be balanced by a cluster of positively charged amino acids in order to allow membrane insertion. (iii) In general, a signal sequence can be sufficient to mediate membrane insertion independently of SRP and docking protein in the case of short precursor proteins; however, the presence and distribution of charged amino acids within the mature part of these precursors can play distinct roles

Biogenesis of Glyoxysomes

Biosynthesis of isocitrate lyase, a tetrameric enzyme of the glyoxysomal matrix, was studied in Neurospora crassa, in which the formation of glyoxysomes was induced by a substitution of sucrose medium by acetate medium. * 1. Translation of Neurospora mRNA in reticulocyte lysates yields a product which has the same apparent molecular weight as the subunit of the functional enzyme. Using N-formyl[35S]methionyl tRNAMetf as a label, the translation product shows the same apparent size which indicates that the amino terminus has no additional 'signal'-type sequence. * 2. Read-out systems employing free and membrane-bound polysomes show that only free ribosomes are active in the synthesis of isocitrate lyase. * 3. Isocitrate lyase synthesized in reticulocyte lysate is released into the supernatant and is soluble in a monomeric form. It interacts with Triton X-100 to form mixed micells in contrast to the functional tetrameric form. * 4. Transfer of isocitrate lyase synthesized in vitro into isolated glyoxysomes is suggested by results of experiments in which supernatants from reticulocyte lysates are incubated with a particle fraction isolated from acetate-grown cells. No transfer occurs when particles from non-induced cells are employed. Resistance to added proteinase is used as a criterion for transmembrane transfer. The data support a post-translational transfer mechanism for isocitrate lyase. They suggest that isocitrate lyase passes through a cytosolic precurscr pool as a monomer and is transferred into glyoxysomes

Kinetic Studies on the Transport of Cytoplasmically Synthesized Proteins into the Mitochondria in Intact Cells of Neurospora crassa

The transport of cytoplasmically synthesized mitochondrial proteins was investigated in whole cells of Neurospora crassa, using dual labelling and immunological techniques. In pulse and pulse-chase labelling experiments the mitochondrial proteins accumulate label. The appearance of label in mitochondrial protein shows a lag relative to total cellular protein, ribosomal, microsomal and cytosolic proteins. The delayed appearance of label was also found in immunoprecipitated mitochondrial matrix proteins, mitochondrial ribosomal proteins, mitochondrial carboxyatractyloside-binding protein and cytochrome c. Individual mitochondrial proteins exhibit different labelling kinetics. Cycloheximide inhibition of translation does not prevent import of proteins into the mitochondria. Mitochondrial matrix proteins labelled in pulse and pulse-chase experiments can first be detected in the cytosol fraction and subsequently in the mitochondria. The cytosol matrix proteins and those in the mitochondria show a precursor-product type relationship. The results suggest that newly synthesized mitochondrial proteins exist in an extra-mitochondrial pool from which they are imported into the mitochondria

Biosynthetic pathway of mitochondrial ATPase subunit 9 in Neurospora crassa

Subunit 9 of mitochondrial ATPase (Su9) is synthesized in reticulocyte lysates programmed with Neurospora poly A-RNA, and in a Neurospora cell free system as a precursor with a higher apparent molecular weight than the mature protein (Mr 16,400 vs. 10,500). The RNA which directs the synthesis of Su9 precursor is associated with free polysomes. The precursor occurs as a high molecular weight aggregate in the postribosomal supernatant of reticulocyte lysates. Transfer in vitro of the precursor into isolated mitochondria is demonstrated. This process includes the correct proteolytic cleavage of the precursor to the mature form. After transfer, the protein acquires the following properties of the assembled subunit: it is resistant to added protease, it is soluble in chloroform/methanol, and it can be immunoprecipitated with antibodies to F1-ATPase. The precursor to Su9 is also detected in intact cells after pulse labeling. Processing in vivo takes place posttranslationally. It is inhibited by the uncoupler carbonylcyanide m- chlorophenylhydrazone (CCCP). A hypothetical mechanism is discussed for the intracellular transfer of Su9. It entails synthesis on free polysomes, release of the precursor into the cytosol, recognition by a receptor on the mitochondrial surface, and transfer into the inner mitochondrial membrane, which is accompanied by proteolytic cleavage and which depends on an electrical potential across the inner mitochondrial membrane

Studies of Impefection Sensitive Conical Composite Structures

The stability of shell structures has been an object of studies for more than a century. Thin walled cylindrical and conical structures are widely used in aerospace, offshore, marine, civil and other industries. Nowadays, with the growing application of composite materials a deep understanding of the influence of their properties and the laminate stacking sequence on the mechanical behaviour of shell structures is increasingly more important. As it is already known, one of the most significant sources of discrepancy between theoretical predictions and experimental results for the buckling load is the presence of geometric imperfections. Currently, imperfection sensitive shell structures are generally designed, at the preliminary design phase, according to the guideline NASA SP-8007 for cylinders and NASA SP-8019 for truncated cones using the conservative lower bound curve, which does not consider composite material characteristics. Hühne developed the Single Perturbation Load Approach (SPLA), a robust design method that stimulates a single buckle, which is assumed as a “worst-case” geometrical imperfection [1]. There have been carried out considerably more numerical, analytical and experimental studies on cylindrical shells than on conical shells. Currently typical composite launcher structures are investigated by 12 partners in the European project DESICOS [4]. The aim of this paper is to study the SPLA on a conical shell structure and compare it with the NASA design approach

Probabilistic Approach for better Buckling Knock-down Factors of CFRP Cylindrical Shells - Tests and Analyses

The industry in the fields of civil and mechanical engineering, and in particular of aerospace demands for significantly reduced development and operating costs. Reduction of structural weight at safe design is one avenue to achieve this objective. The running ESA (European Space Agency) study Probabilistic Aspects of Buckling Knock Down Factors – Tests and Analyses contributes to this goal by striving for an improved buckling knock-down factor (the ratio of buckling loads of imperfect and perfect structures) for unstiffened CFRP (carbon fiber reinforce plastics) cylindrical shells, and by validation of the linear and non-linear buckling simulations based on test results. DLR is acting as study contractor. The paper presents an overview about the DLR buckling tests, the measurement setup and the buckling simulations which are done so far, and gives an outlook to the results which are expected until the end of the running project