A GTPase reaction accompanying the rejection of Leu-tRNA2 by UUU-programmed ribosomes. Proofreading of the codon-anticodon interaction by ribosomes

The characteristics of a GTPase reaction between poly(U)-programmed ribosomes, EFTu . GTP, and the near-cognate aminoacyl (aa)-tRNA, Leu-tRNA Leu 2, have been studied to assess the role of this reaction in proofreading of the codon-anticodon interaction. The reaction resembles the GTPase reaction with cognate aa-tRNAs and EFTu . GTP in its substrate requirements, in its involving EFTu . GTP . aa-tRNA ternary complexes, and in its requiring a free ribosomal A-site. The noncognate reaction differs from the cognate one in that aa-tRNA becomes stably bound to the ribosomes only 5% of the time; it therefore seems best characterized as an abortive enzymatic binding reaction. The rate of reaction is a significant fraction (4%) of that of the cognate aa-tRNA, indicating that recognition of ternary complexes by ribosomes involves a level of error greater than that of translation as a whole. The rejection of the noncognate aa-tRNA following GTP hydrolysis is therefore a vital step in the translation process and fulfills the criteria set for a proofreading reaction. Leu-tRNA Leu 2 which escapes rejection through proofreading, forms a stable complex with the ribosomal A-site, so it appears that the Leu-tRNA2 which was rejected never reached the A-site and that proofreading precedes full A-site binding.     

Characterization of sodium transport in gustatory epithelia from the hamster and rat

The transduction of sodium salts occurs through a variety of mechanisms, including sodium influx through amiloride-sensitive sodium channels, anion-dependent sodium movement through intercellular junctions and unidentified amiloride-insensitive mechanisms. Characterizations of sodium transport in lingual epithelium mounted in Ussing chambers have focused almost exclusively on epithelia containing only fungiform taste buds. In the present study we have investigated sodium transport by measuring NaCl-induced short-circuit current from lingual epithelia containing fungiform, foliate, vallate and palatine taste buds in the hamster and the rat. All areas show measurable sodium transport, yet significant differences were noted between the epithelia from the rat and the hamster and among the different epithelia within a single species in terms of current density, transepithelial resistance and mucosal amiloride sensitivity. In general, epithelia from the anterior tongue were of a lower resistance and transported sodium more effectively than from the posterior tongue. Moreover, fungiform- and vallate-containing epithelia in the rat had a greater current density than did the corresponding tissues in the hamster. Amiloride sensitivity also differed between the rat and the hamster. In the hamster all gustatory areas showed some amiloride sensitivity, while in the rat the vallate-containing epithelia were devoid of amiloride- sensitive sodium transport. The results are consistent with the interpretation that all chemosensitive areas may participate in the detection of salts but the degree of salt transport and the mechanism of transport is variable among different lingual epithelia and different species.      

The incidence of hematologic tumours: A cellular model for the age dependence

Most hematologic tumors (acute and chronic myelogenous leukemia, chronic lymphatic leukemia, polycythemia vera, and multiple myeloma) exhibit an exponential increase in incidence with advancing age of the host. This age-incidence pattern resembles that for carcinomas and can be explained by the accumulation of harmful mutations in the stem cells of the tissues of tumor origin during the course of normal aging. The age-incidence pattern for acute lymphatic leukemia is unique and complex with a linear increase in incidence from birth to age 3, an exponential decline in incidence form age 3 to 34, a low and constant rate of incidence from age 34 to 60, and a slight increase in incidence at ages greater than 60. We conclude that variation in tumor incidence with host age is determined by the pattern of cell proliferation in the tissue of tumor origin. We suggest that cell proliferation in the tissue of origin of acute lymphatic leukemia occurs in stages: (1) rapid stem cell proliferation from before birth to age 3, (2) random stem cell differentiation from age 3 to 34, and (3) a constant rate of stem cell proliferation and differentiation after age 34.     

Double staining technique for rat foetus skeletons in teratological studies.

Cartilage and bone were differentiated using alcian blue and alizarin red S respectively. Anomalies of both cartilaginous and bony parts of the skeleton could be examined.     

Design rationale, synthesis, and characterization of non-natural analogs of the cationic amino acids arginine and lysine.

A series of non-natural isosteric analogs of the cationic, ion-pairing, natural amino acids arginine and lysine have been synthesized, characterized with regard to relevant physical parameters, and protected for routine inclusion in Merrifield solid-phase synthesis. The design of these molecules is based on the concept of steric inhibition of solvation, in that judicious placement of alkyl groups can destabilize aqueous ion solvation and favor ion-pairing [see Beeson & Dix (1993) J. Am. Chem. Soc. 115, 10275]. When the residues are substituted for the natural amino acids in biologically active peptides, enhanced ion-pairing of the peptides to their receptors to increase the peptides' biological activities can result. The increased lipophilicity of the non-natural residues can also improve pharmacokinetic parameters and agonist/antagonist behaviors of peptides. While the synthesis of the L-series is described, the D-isomers were also prepared using identical chemistry.     

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.