Possible role in uptake of water vapour by ixodid tick salivary glands

The mouth is confirmed as the site of water vapor uptake in the lone star tick, Amblyomma americanum. It was shown that the level of chloride (³⁶Cl) increased in the mouthparts of desiccated ticks. The highest levels of ³⁶Cl were found in the mouthparts, salivary glands, and gut tissue during rehydration. It is suggested that ions are secreted by the salivary glands into the mouth where water is picked up hygroscopically by the secretion. It is further suggested that the water and ions are then swallowed and absorbed from the lumen of the gut.     

Studies of the structure of the T4 bacteriophage tail sheath: I. The recovery of three-dimensional structural information from the extended sheath

The extended tail sheath of bacteriophage T4 has been used to study the transfer of information from an electron micrograph to the three-dimensional reconstruction obtained from it. Two methods have been developed to assess micrograph images of helical particles and their reconstructions. First, a filter has been designed which eliminates all structure in the image inconsistent with the symmetry and assumed radius of the helical particle. Individual micrographs can therefore be assessed with respect to their consistency with the assumed symmetry and radius, before reconstruction. Second, a map of the root-meansquare deviation of individual reconstructions from their average provides a quantitative measure of the consistency of the individual sets of tail data and allows the regions in the average reconstruction which are most sensitive to differences between the particles to be identified.The averaged reconstruction is used to examine the problems related to resolution and reproducibility of the structural information and to define the extent of the different components of the extended sheath.     

High resolution X-ray structure of yeast hexokinase,an allosteric protein exhibiting a non-symmetric arrangement of subunits

The structure determination of yeast hexokinase has been extended to 3.5 Å resolution for the dimer and to 2.7 Å resolution for the monomer using multiple isomorphous replacement. The electron density maps of both the monomer and dimer crystal forms have been substantially improved by an averaging procedure. From these maps the course of the polypeptide backbone and some aspects of the dimer interaction have been established.The hexokinase subunit arrangement is contrary to a major tenet of the Monod et al. (1965) theory of allosteric proteins which postulated that only symmetric or isologous interactions of subunits would occur in oligomeric proteins. One subunit of the dimer is related to the other by a 156 ° rotation about and a 13.8 Å translation along a molecular screw axis. In the hexokinase dimer the set of residues in one subunit that is interacting with the other subunit is different from the set of residues in the second subunit that is interacting with the first subunit. This heterologous or non-symmetric interaction of subunits is associated with some small differences in the structure of the two subunits, particularly at the subunit interface, and accounts for some of this enzyme's non-symmetric interactions with substrates and activators. Indeed, the non-symmetric subunit association may play an important role in the control of this enzyme's activity.The overall structure of hexokinase is considerably different than the known structures of the other enzymes in the glycolytic pathway. Although there is a striking similarity between the domain of hexokinase that binds AMP and the domain of lactate dehydrogenase that binds NAD, the former structure contains both antiparallel and parallel β-pleated strands, while the latter contains only parallel β-structure. In an attempt to assess the significance of this structural similarity, the structure of the nucleotide binding domains of hexokinase and lactate dehydrogenase are compared to a portion of carboxypeptidase A. The observed similarities among these structures suggests that a central β-pleated sheet flanked by α-helices is a common supersecondary structure that probably arose by convergent as well as divergent evolution. Thus, there appears to be no compelling evidence at this time to support the hypothesis that a part of hexokinase has evolved from the same gene as the dinucleotide binding domain of lactate dehydrogenase.     

The fine structure of a microplate-microtubule array,microfilaments and polyhedral body associated microtubules in several species of Anabaena

A microplate-microtubule array was observed in Anabaena sp. (B-378). This structure consists of an arched plate, about 8 nm thick, and various microtubules, 12 nm in diameter and 50 nm long, arranged in rows. The microtubules project at right angles from one side of the plate into the cytoplasm or towards the plasma membrane. Up to twelve microplate-microtubule arrays were observed in a single section of a cell.Microfilaments, about 2.8 nm in diameter and of undetermined length, were observed in four isolates of Anabaena. The microfilaments were always found in bundles, which varied in size, up to 0.63 m across and 0.91 long.Microtubules, 10 nm in diameter and about 150 nm in length, were observed associated with one facet of polyhedral bodies in 8 out of 20 isolates of Anabaena. The microtubules occurred in groups of up to 20 or more, and were always oriented with the long axis parallel to a facet of a polyhedral body. In cross section, the microtubules had an electron transparent lumen 5 nm wide and a wall 2.5 nm thick.These structures are compared to previously deseribed microtubules and microfilaments.     

Neural correlates of frog calling

Summary A tissue bath incorporating a screen for support of the specimen and an air-lift pump to circulate saline across the screen was designed to provide maximum exposure of isolated frog brainstems ofRana pipiens pipiens to oxygenated saline (Fig. 1). Normal neural correlates of electrically-evoked mating calling were recorded from the region of the pretrigeminal nucleus and the laryngeal nerve in the isolated brainstem (Fig. 3A) and isolated hemi-brainstem (Fig. 2) of the Northern leopard frog. Conspicuous slow-wave activity in the region of the pretrigeminal nucleus supports the possibility that this may be an important integrative area for calling. It appears that the pretrigeminal region is not able, independently, to generate the pulses of the vocal phase of calling. Synchronizing and reinforcing inter-connections between the calling mechanisms of the two sides were identified. The data are summarized in a revised model of mating calling (Fig. 7).This work was supported by NINDS grant NS-06673. The electronic equipment was set up and maintained by Mr. Wayne R. Hudson. I am grateful to Dr. William Van Meter for suggesting the Sylgard for the pinning block and to Dr. Patricia Gallagher for suggesting the saline solution of Phillis and Tebcis.     

Auditory sensitivity of the saccule in the American toad (Bufo americanus)

Summary Single unit recordings in the posterior nerve branchlet from the saccule have shown that, in the American toad (Bufo americanus), approximately 30% of the fibers respond to airborne sounds in a way similar to fibers from the two known auditory organs, the amphibian and basilar papillae. In response to tones, saccule fibers have best excitatory frequencies which fall into two disjoint populations: units in the low-frequency-sensitive group (below 300 Hz) show tone-on-tone suppression while those in the high-frequency-sensitive group (700–1,200 Hz) show no evidence of peripheral inhibition. Saccule units have somewhat higher thresholds than those from the other auditory organs. It is suggested that the high-frequency-sensitive fibers might be useful for discriminating mating calls in an intense chorus while the low-frequency-sensitive units likely respond to other high intensity sounds in the environment.Research supported by the U.S. Public Health Service (NIH grant NS-09244).     

Purification,properties, and mode of action of hemicellulase I produced by Ceratocystis paradoxa

A culture isolate (CP₂) of the fungal plant pathogen Ceratocystis paradoxa produces at least five extra-cellular hemicellulases when grown on a medium containing a commercial hemicellulose as inducer. One of the five enzymes, hemicellulase I (HC-I), was purified by ammonium sulphate precipitation, ion-exchange chromatography (DEAE-Sephadex and then Cellex-CM), and iso-electric focusing at pH 3–10 and 8–10. HC-I behaves as a single protein on electrophoresis at pH 6.0 and 8.4. The enzyme degrades hemicellulose B (an arabino-4-O-methylglucuronoxylan) and arabinoxylan to arabinose, xylose, xylobiose (Xyl₂; β-D-Xylp-(1→4)-D-Xyl), and a mixture of arabinose-xylose and xylose oligosaccharides (AraXyl_n and Xyl_n, where n  3, 4, or 5). The enzyme is deduced to be an endo-enzyme. Xylotetraose (Xyl₄) was the lowest homologue of the xylose oligosaccharides attacked, yielding xylobiose and xylotriose (Xyl₃) only. A mechanism is postulated for this reaction. AraXyl₂AraXyl₅ were slowly hydrolysed to arabinose and the respective xylose saccharide (Xyl₂Xyl₅), and thence to Xyl₂ and Xyl₃. Hydrolysis of the arabinofuranosyl linkage probably does not occur at the same active site as for the xylose oligosaccharides. Hemicellulose B fractions from different sources appeared to be degraded by HC-I. The enzyme showed optimum activity at pH 5.5 and 40°, and K_m was 4.24 mg of hemicellulose/ml.