The effect of spherical confinement on the ground state of helium-like ions (Z = 1-5) and the first four ( 1 S e ) excited states for Z = 2-5 have been analyzed in detail by using correlated Hylleraas basis sets within the variational framework. The correlated wave functions used here are consistent with the finite boundary conditions due to spherical confinement. The present energy values and the thermodynamic pressure generated for the confined ions as well as the critical values of the confining radii close to the fragmentation limit can be set as a benchmark for future references.

The mitochondrial genomes of a wide variety of species contain an insufficient number of functional tRNA genes, and translation of mitochondrial mRNAs is sustained by import of nucleus-encoded tRNAs. In Leishmania, transfer of tRNAs across the inner membrane can be regulated by positive and negative interactions between them. To define the factors involved in such interactions, a large multisubunit complex (molecular mass, ϳ640 kDa) from the inner mitochondrial membrane of the kinetoplastid protozoon Leishmania, consisting of ϳ130-Å particles, was isolated. The complex, when incorporated into phospholipid vesicles, induced specific, ATP-and proton motive force-dependent transfer of Leishmania There is remarkable diversity in the scope and mechanism of mitochondrial tRNA import (reviewed in reference 18). Human mitochondria do not import tRNA, but a number of neuromuscular degenerative and metabolic diseases are caused by mutations in mitochondrial tRNA genes (21). In yeast, a single tRNA is imported, apparently through protein import channels and requiring at least two soluble factors, including the mitochondrial form of the cognate aminoacyl-tRNA synthetase (8). By contrast, in kinetoplastid protozoa (Leishmania and trypanosomes), import of a whole spectrum of tRNAs is necessitated by the complete lack of mitochondrial tRNA genes (5,19). In this system, membrane-bound tRNA binding proteins recognize specific structural motifs (import signals) on tRNA, soluble factors are not required, and the translocation pathway appears to be distinct from that for protein import (11,14,17). Moreover, the sequence and bioenergetic requirements for outer and inner membrane transfer are nonidentical (2), indicating the presence of a distinct transport machinery (the RNA import complex [RIC]) at the inner membrane, a situation similar to the TOM and TIM complexes for protein import (15). A 15-kDa polypeptide has been shown to be required for import into Leishmania mitochondria (1); otherwise, the import machinery remains undefined.Using an in vitro evolution protocol, it was recently shown that Leishmania mitochondria recognize a number of short sequence motifs homologous to multiple domains in tRNAs, suggesting the presence of several import signals (3). Moreover, novel positive and negative allosteric interactions between these aptamers, as well as between intact tRNAs, at the inner membrane were described (3). The RNAs could be classified into two types: type I RNAs are efficiently transferred through the inner membrane but are inhibited by type II. In contrast, type II RNAs have poor inner membrane transfer efficiencies and are stimulated by type I. For example, tRNA Tyr (GUA) is a type I RNA containing the conserved motif UA GAGC in the D domain, while tRNA Ile (UAU) is type II with the sequence UCGCGGGUU in the variable loop-T domain (V-T) region (3). The mechanism of these allosteric interactions is unknown, but there are several possibilities. A single conformationally flexible dimeric or multimeric receptor could bind t...

A large number of cytoplasmic tRNAs are imported into the kinetoplast-mitochondrion of Leishmania by a receptor-mediated process. To identify the sequences recognized by import receptors, mitochondria were incubated with a combinatorial RNA library. Repeated cycles of amplification of the imported sequences (SELEX) resulted in rapid selection of several import aptamers containing sequence motifs present in the anticodon arm, the D arm, the V-T region, and acceptor stem of known tRNAs, confirming or suggesting the presence of import signals in these domains. As predicted, truncated derivatives of tRNA Ile (UAU) containing the D arm or the V-T region were imported in vitro. Four aptamers were studied in detail. All were imported in vitro as well as in transiently transfected cells, using the same pathway as tRNA, but their individual import efficiencies were different. Two types of aptamers were discernible: the A arm and D arm homologues (type I), which were efficiently transferred across the inner mitochondrial membrane, and the V-T homologues (type II), which were not. Remarkably, subnanomolar concentrations of type I RNAs stimulated the entry of type II RNAs into the matrix, whereas type II RNAs inhibited inner membrane transfer of type I RNAs. Moreover, tRNA Tyr (GUA) and tRNA Ile (UAU) interacted with one another as type I and type II, respectively. Such cooperative and antagonistic interactions may allow the use of a limited number of receptors to recognize a large number of tRNAs of variable affinity and enable the maintenance of a properly balanced tRNA pool for mitochondrial translation.

Plumbagin, a plant-derived bioactive naphthoquinonoid compound, was converted to a hydroquinonoid derivative, which was studied for its tumour-inhibitory and antileishmanial activities for the first time. A similar chemical transformation was undertaken on an analogous dimeric compound, diospyrin, and its bioassay results were compared with those of the plumbagin derivative. Synthesis of the derivative of plumbagin did not result in a marked enhancement of the tumour-inhibitory activity, whereas the improvement was obvious in the case of diospyrin vis à vis its hydroquinonoid analogue. The conversion of diospyrin to the hydroquinonoid compound also led to a substantial increase in the antileishmanial activity, while a similar conversion of plumbagin failed to do so.

Import of tRNA into Leishmania mitochondria involves transfer through a double membrane barrier. To examine whether specific sorting mechanisms for individual tRNAs direct them to different mitochondrial compartments, the distribution of tRNA transcripts, internalized in vitro, was examined by suborganellar fractionation. Significant amounts of tRNA Tyr were localized in the matrix and on the outer face of the inner mitochondrial membrane. With time, the matrix:membrane ratio increased. Translocation through the inner membrane apparently required the presence of a specific signal in the D arm of tRNA Tyr , and tRNA Gln (CUG), lacking this sequence, was excluded. Hydrolysis of ATP was necessary at both the outer and inner membranes. However, the protonophores carbonylcyanide m-chlorophenylhydrazone and nigericin, the K ؉ ionophore valinomycin, and the F 1 F 0 ATPase inhibitor oligomycin had only marginal effects on uptake through the outer membrane but severely inhibited inner membrane translocation, indicating the unusual requirement of both the electrical and chemical components of the electromotive force generated across the inner membrane. The results are consistent with a mechanism involving stepwise transfer of tRNA through distinct outer and inner membrane channels.