Mechanism and dynamics of conformational ordering in xanthan polysaccharide

The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.     

Protists have divergent effects on bacterial diversity along a productivity gradient

Productivity and predation are thought to be crucial drivers of bacterial diversity. We tested how the productivity–diversity of a natural bacterial community is modified by the presence of protist predators with different feeding preferences. In the absence of predators, there was a unimodal relationship between bacterial diversity and productivity. We found that three protist species (Bodo, Spumella and Cyclidium) had widely divergent effects on bacterial diversity across the productivity gradient. Bodo and Cyclidium had little effect on the shape of the productivity–diversity gradient, while Spumella flattened the relationship. We explain these results in terms of the feeding preferences of these predators.     

Trypsin-sensitive neutralization site on VP1 of Theiler''s murine encephalomyelitis viruses.

We generated Theiler's murine encephalomyelitis virus mutants resistant to several neutralizing monoclonal antibodies (MAbs) having their epitopes near a trypsin cleavage site of VP1. Neutralization and Western blot (immunoblot) studies suggest that two of the MAbs have identical epitopes that partly overlap the epitope of a third MAb. Sequencing of RNA of the mutants localized the epitopes to a site near the carboxyl end of VP1. The limited diversity of nucleotide changes seen in the mutants and the immunodominance of the site suggest that the carboxyl end of VP1 may have an important function.     

Prothymosin alpha and parathymosin: amino acid sequences deduced from the cloned rat spleen cDNAs

A rat spleen cDNA library was screened for clones carrying the cDNAs for prothymosin alpha and parathymosin. Sequence analysis of a clone carrying the entire coding region for prothymosin alpha confirmed and completed the amino acid sequence for this polypeptide and established the number of amino acid residues as 111. Rat prothymosin alpha differs from human prothymosin alpha at six positions, including four substitutions and two insertions. The nucleotide sequences of the cDNAs for the rat and human polypeptides are more than 90% identical in the open reading frames, with significant homology extending into the 5' and 3' flanking regions. From the same library, we also isolated a clone carrying 80% of the coding region for rat parathymosin. The number of amino acid residues in rat parathymosin is 101, based on the sequence deduced from the cDNA insert and earlier information on the sequence in the amino-terminal portion of this polypeptide. Despite their similarity in size and amino acid composition, rat prothymosin alpha and rat parathymosin show only limited sequence homology, primarily in the segment including residues 14 through 25, where 10 of 12 positions are identical in the two polypeptides. this is also the region of significant sequence similarity to a 12-amino-acid segment in the p17 protein of the human immunodeficiency disease associated virus (HTLV-IIIB).