Generation of a locomotory rhythm by a neural network with recurrent cyclic inhibition

An oscillatory intersegmental neuronal network drives the swimming rhythm of the leech. This network consists of interneurons joined via inhibitory connections to form a series of segmentally iterated, concatenated rings. Recurrent cyclic inhibition in these rings produces a multiphasic activity rhythm. By theoretical analysis of such concatenated interneuronal rings and construction of their electronic analogs it is shown that the interneural network identified in the central nervous system of the leech has properties appropriate for generating the observed motor output.     

Subject index to volume 32

Subject Index

Subject index to volume 32     

Large, rapidly evolving intergenic spacers in the mitochondrial DNA of the salamander family Ambystomatidae (Amphibia: Caudata)

We report the presence, in the mitochondrial DNA (mtDNA) of all of the sexual species of the salamander family Ambystomatidae, of a shared 240- bp intergenic spacer between tRNAThr and tRNAPro. We place the intergenic spacer in context by presenting the sequence of 1,746 bp of mtDNA from Ambystoma tigrinum tigrinum, describe the nucleotide composition of the intergenic spacer in all of the species of Ambystomatidae, and compare it to other coding and noncoding regions of Ambystoma and several other vertebrate mtDNAs. The nucleotide substitution rate of the intergenic spacer is approximately three times faster than the substitution rate of the control region, as shown by comparisons among six Ambystoma macrodactylum sequences and eight members of the Ambystoma tigrinum complex. We also found additional inserts within the intergenic spacers of five species that varied from 87-444 bp in length. The presence of the intergenic spacer in all sexual species of Ambystomatidae suggests that it arose at least 20 MYA and has been a stable component of the ambystomatid mtDNA ever since. As such, it represents one of the few examples of a large and persistent intergenic spacer in the mtDNA of any vertebrate clade.      

Movement of ribosomes over messenger RNA in polysomes of rel + and rel - Escherichia coli strains

The in vitro movement of ribosomes over messenger RNA was studied in both the presence and the absence of protein synthesis. For this purpose, labeled polysomes were extracted from rel⁺ and rel^? strains of Escherichia coli grown in the presence of radioactive uracil and incubated in a cell-free system containing tRNA, amino acids, soluble enzymes and a source of energy. The gradual conversion of the labeled polysomes into monosomes and ribosomal subunits was followed by subjecting the reaction mixture to sucrose gradient sedimentation after various incubation times and measuring the radioactivity present in the three relevant ribosomal fractions.It was found that when the conditions of incubation allow protein synthesis to occur, polysomes extracted from rel⁺ and rel^? cells are converted mainly into free monosomes, which can be made to dissociate into subunits by high-sodium or low-magnesium ion concentrations. Under conditions in which protein synthesis cannot occur because a mutant aminoacyl-tRNA synthetase has been rendered inactive, polysome conversion still occurs, though to a reduced extent. When the products of such residual run-off are examined, however, a difference is manifest between polysomes extracted from rel⁺ and from rel^? strains: whereas the polysomes from the rel^? strain are still converted into free monosomes even in the absence of protein synthesis, the polysomes from the rel⁺ strain are now converted mainly into subunits. It can be inferred, therefore, that ribosomes from rel^? bacteria, but not those from rel⁺ bacteria, continue movement over messenger RNA in the absence of protein synthesis.Studies of mixed extracts from rel^? and rel⁺ bacteria have shown that the character of the run-off process does not depend on the source of tRNA and soluble enzymes; the proportions of monosomes and subunits among the run-off products formed in the absence of protein synthesis depend only on the source of the polysomes. It is suggested that the mutation of the rel gene alters the functional architecture of ribosomes.     

Specificity of the amino acid transfer reaction inE. coli strains carrying different alleles of the RNA control (RC) gene

Summary The idea has been tested here that the aberration in amino acid controlled regulation of RNA synthesis in a mutant strain ofE. coli might reflect a major breakdown in the specificity of transfer of amino acids to S-RNA. For this purpose, S-RNA and amino acid activating enzymes were extracted from bacteria carrying either the normalRC ^st or the aberrantRC ^rel allele of the RNA control gene. The purified S-RNA preparations were first charged enzymatically with one or more of the 20 standard amino acids, then oxidized with periodate, and finally reisolated and retested for their residual capacity to accept an amino acid that was absent from the preliminary charging mixture. If preliminary charging transferred an amino acid to a non-cognate S-RNA species belonging to an absent amino acid, then the acceptor capacity for the missing amino acid would survive periodate oxidation and reveal its presence on recharging with that amino acid after post-periodate reisolation of the S-RNA. The results presented here show that there does not appear to exist any such major breakdown of transfer specificity in eitherRC ^st orRC ^rel bacteria: preliminary charging of the S-RNA fromRC ^rel bacteria with 19 of the 20 standard amino acids by use of the homologous amino acid activating enzymes does not afford protection against periodate oxidation for any appreciable fraction of the acceptor capacity for the absent 20th amino acid (when that amino acid is either methionine or arginine). It is unlikely, therefore, that thecatholic inducer, postulated to explain the continued RNA synthesis ofRC ^rel amino acid auxotrophs in the absence of their growth requirement, is one of the 20 standard amino acids.This investigation was supported by Public Health Service Research Grant CA 02129, from the National Cancer Institute.