Major Termination Site Research Articles

Interaction between the Yeast Mitochondrial and Nuclear Genomes Influences the Abundance of Novel Transcripts Derived from the Spacer Region of the Nuclear Ribosomal DNA Repeat

We have identified stable transcripts from the so-called nontranscribed spacer region (NTS) of the nuclear ribosomal DNA repeat in certain respiration-deficient strains of Saccharomyces cerevisiae. These RNAs, which are transcribed from the same strand as is the 37S rRNA precursor, are 500 to 800 nucleotides long and extend from the 5' end of the 5S rRNA gene to three major termination sites about 1,780, 1,830, and 1,870 nucleotides from the 3' end of the 26S rRNA gene. A survey of various wild-type and respiration-deficient strains showed that NTS transcript abundance depended on the mitochondrial genotype and a single codominant nuclear locus. In strains with that nuclear determinant, NTS transcripts were barely detected in [rho+] cells, were slightly more abundant in various mit- derivatives, and were most abundant in petites. However, in one petite that was hypersuppressive and contained a putative origin of replication (ori5) within its 757-base-pair mitochondrial genome, NTS transcripts were no more abundant than in [rho+] cells. The property of low NTS transcript abundance in the hypersuppressive petite was unstable, and spontaneous segregants that contained NTS transcripts as abundant as in the other petites examined could be obtained. Thus, respiration deficiency per se is not the major factor contributing to the accumulation of these unusual RNAs. Unlike RNA polymerase I transcripts, the abundant NTS RNAs were glucose repressible, fractionated as poly(A)+ RNAs, and were sensitive to inhibition by 10 micrograms of alpha-amanitin per ml, a concentration that had no effect on rRNA synthesis. Abundant NTS RNAs are therefore most likely derived by polymerase II transcription.

Interaction between the yeast mitochondrial and nuclear genomes influences the abundance of novel transcripts derived from the spacer region of the nuclear ribosomal DNA repeat.

We have identified stable transcripts from the so-called nontranscribed spacer region (NTS) of the nuclear ribosomal DNA repeat in certain respiration-deficient strains of Saccharomyces cerevisiae. These RNAs, which are transcribed from the same strand as is the 37S rRNA precursor, are 500 to 800 nucleotides long and extend from the 5' end of the 5S rRNA gene to three major termination sites about 1,780, 1,830, and 1,870 nucleotides from the 3' end of the 26S rRNA gene. A survey of various wild-type and respiration-deficient strains showed that NTS transcript abundance depended on the mitochondrial genotype and a single codominant nuclear locus. In strains with that nuclear determinant, NTS transcripts were barely detected in [rho+] cells, were slightly more abundant in various mit- derivatives, and were most abundant in petites. However, in one petite that was hypersuppressive and contained a putative origin of replication (ori5) within its 757-base-pair mitochondrial genome, NTS transcripts were no more abundant than in [rho+] cells. The property of low NTS transcript abundance in the hypersuppressive petite was unstable, and spontaneous segregants that contained NTS transcripts as abundant as in the other petites examined could be obtained. Thus, respiration deficiency per se is not the major factor contributing to the accumulation of these unusual RNAs. Unlike RNA polymerase I transcripts, the abundant NTS RNAs were glucose repressible, fractionated as poly(A)+ RNAs, and were sensitive to inhibition by 10 micrograms of alpha-amanitin per ml, a concentration that had no effect on rRNA synthesis. Abundant NTS RNAs are therefore most likely derived by polymerase II transcription.

Morphology of vertical canal related second order vestibular neurons in the cat.

The morphology of vertical canal related second order vestibular neurons in the cat was studied with the intracellular horseradish peroxidase method. Neurons were identified by their monosynaptic potentials following electrical stimulation via bipolar electrodes implanted into individual semicircular canal ampullae. Anterior and posterior canal neurons projected primarily to contralateral or ipsilateral motoneuron pools (excitatory and inhibitory pathways, respectively). The axons of contralaterally projecting neurons crossed the midline at the level of the abducens nucleus and bifurcated into an ascending and a descending main branch which travelled in the medial longitudinal fasciculus (MLF). Two types of anterior canal neurons were observed, one with unilateral and one with bilateral oculomotor projection sites. For both neuron classes, the major termination sites were in the contralateral superior rectus and inferior oblique subdivisions of the oculomotor nucleus. In neurons which terminated bilaterally, major collaterals recrossed the midline within the oculomotor nucleus to reach the ipsilateral superior rectus motoneuron pool. Other, less extensive, termination sites of both neuron classes were in the contralateral vestibular nuclear complex, the facial nucleus, the medullary and pontine reticular formation, midline areas within and neighboring the raphé nuclei, and the trochlear nucleus. The ascending main axons continued further rostrally to reach the interstitial nucleus of Cajal and areas around the fasciculus retroflexus. The descending branches proceeded further caudal in the medial vestibulo-spinal tract but were not followed to their spinal target areas. In addition to two previously described posterior canal related neuron types (Graf et al. 1983), we found neurons with bilateral oculomotor terminals and a spinal collateral. Typical for posterior canal neurons, the major termination sites were in the trochlear nucleus (superior oblique motoneurons) and in the inferior rectus subdivision of the oculomotor nucleus. Axon collaterals recrossed the midline to reach ipsilateral inferior rectus motoneurons. The axons of ipsilaterally projecting neurons ascended through the reticular formation to join the MLF caudal to the trochlear nucleus. The main target sites of anterior canal related neurons were in the trochlear nucleus and the inferior rectus subdivision of the oculomotor nucleus. Minor collaterals reached the pontine reticular formation and areas in between the fiber bundles of the ipsilateral MLF.(ABSTRACT TRUNCATED AT 400 WORDS)

The vestibuloocular reflex of the adult flatfish. II. Vestibulooculomotor connectivity.

The peripheral and central oculomotor organization of the adult flatfish presents no morphological substrates that suffice to explain adaptive changes in its vestibuloocular reflex system. The necessity for an adaptation occurs because of a 90 degrees displacement of the vestibular with respect to the extraocular coordinate axes during metamorphosis. Since a reorganization of vestibuloocular pathways must be hypothesized (12), the location and termination of electrophysiologically identified secondary vestibular neurons with focus on the horizontal canal system was studied with the intracellular horseradish peroxidase method in adult winter flounders. Pseudopleuronectes americanus. The oculomotor target sites of vertical canal related neurons were similar to those described in mammals. Presumed excitatory anterior canal neurons bifurcated after the main axon had crossed the midline. The descending branch headed toward the spinal cord. The ascending branch reached the oculomotor nucleus via the contralateral medial longitudinal fasciculus and terminated in the superior rectus and inferior oblique subdivisions. Presumed inhibitory posterior canal neurons ascended ipsilaterally in the medial longitudinal fasciculus and terminated mainly in the superior rectus and inferior oblique subdivisions. Horizontal canal neurons exhibited characteristics distinctly different from mammalian ones. Two types of second-order neurons were observed. In the first case, cell bodies were located in the anterior portion of the vestibular nuclear complex. After crossing the midline, the axon ascended in the contralateral medial longitudinal fasciculus. Major termination sites were found in the inferior oblique and superior rectus subdivisions of the oculomotor nucleus. Axonal branches then recrossed the midline and terminated in identical locations on the ipsilateral side. In the second case, cell bodies were located in the descending vestibular nucleus. Their axons crossed the midline and also ascended in the contralateral medial longitudinal fasciculus. Major termination sites were in the trochlear nucleus and in the inferior rectus subdivision of the oculomotor nucleus. As in the first case, axonal branches also recrossed the midline and terminated in identical motoneuron pools on the ipsilateral side. The above target sites were exactly those expected to be used in a reciprocal excitatory-inhibitory fashion during compensatory eye movements. Head-down movement would be excitatory for the lower horizontal canal producing contractions of both superior recti and inferior obliques as well as relaxation of the antagonistic inferior recti and superior obliques.(ABSTRACT TRUNCATED AT 400 WORDS)

Maturation of Escherichia coli tryptophan operon mRNA: evidence for 3' exonucleolytic processing after rho-dependent termination.

The mature 3' end of Escherichia coli tryptophan operon mRNA in vivo coincides with a site (trp t) having features commonly associated with rho-independent terminators in bacteria. Efficient generation of this 3' end in vivo is nevertheless affected by a distal rho-dependent site (trP-t'), though these two sites behave independently in vitro. We have cloned these sites upstream of the galactokinase gene (galK), and galactokinase levels in vivo indicate that, as terminators per se, their efficiencies (37% for trp t, and 79% for trp t') do not differ significantly from those observed in vitro. However, when the trp t hairpin is placed between galK and a downstream copy of trp t', galactokinase levels are enhanced 2- to 3-fold. This suggests the involvement of a post-transcriptional event, such as RNA processing, in determining the level of gene activity. Indeed, in the presence of the 3' exonuclease RNase II, mRNA terminated by rho factor in vitro at the trp t' site is processed back to the trp t site. The remote trp t' region appears to be the major termination site for trp mRNA, and the trp t hairpin serves a dual function-as a minor terminator, and as a protective barrier to 3' exonucleolytic degradation. We infer that the tandem terminators, rho factor, and RNA processing are all required to generate the mature 3' end of this bacterial mRNA.

The tyrT locus: Termination and processing of a complex transcript

The tyrT locus of E. coli contains a 208 bp spacer region that separates two copies of sequence encoding tRNA 1 Tyr. The spacer includes a 120 bp sequence that is homologous to a sequence that is repeated three times in the distal portion of the tyrT locus. The tyrT locus possesses a graded set of transcription termination sites that are spaced at 180 base intervals, corresponding to the distal repeated gene structure. The major termination site occurs within the second repeat unit, 225 bases beyond the mature tRNA sequences. In the presence of a temperature-sensitive rho protein there is increased read-through at this site to a termination site located 180 bases downstream in the third repeat and to several termination sites even further downstream. The primary native transcript, in the region distal to the second tRNA, carries the information for a low molecular weight, extremely basic protein. Although analogous coding sequences are present in the spacer and other repeat units, because of single base substitutions these sequences are pseudogenes. The parallel between the tyrT and tyrU gene clusters is discussed in relation to dual function transcripts that specify both tRNA and protein.

Transcription termination sites in the major leftward operon of coliphage lambda

A transcriptional mapping technique was employed to determine the locations of leftward transcription termination sites within the p L- att region of bacteriophage λ. Lysogens in which phage or host mutations blocked N-mediated antitermination were used. The results are consistent with the presence of three major termination sites, designated t L1, t L2, and t L3, which map at about 71.1 (between genes N and ral), 68.9 (likely within gene cIII) and 64.4 (within gene exo) %λ units, respectively, measured from the left terminus of the mature λ DNA. The t L1 terminator is about 80% efficient, while the t L2 and t L3 terminators are over 90% efficient. The locations of other possible terminators, including one near the nutL site and another within the int gene, are also discussed.

Second order auditory pathways in the chimpanzee.

Substantial portions of the dorsal, and almost the entire posteroventral and anteroventral (Av) cochlear nuclei were aspirated unilaterally in a chimpanzee. Axonal degeneration was studied by the Fink-Heimer method. The greatest amount of degeneration was followed medially from the region of Av into the lateral part of the trapezoid body. Degeneration also coursed around the superior surface of the restiform body and was traced into the dorsal and intermediate acoustic striae. Within the superior olivary complex, degeneration was distributed to: the ipsilateral lateral superior olive; laterally and medially oriented dendrites of the ipsilateral and contralateral medial superior olivary nuclei respectively (some perisomatic degeneration also was present bilaterally); the contralateral medial trapezoid nucleus; retro-olivary and preolivary cell groups bilaterally. Abundant degeneration passed into the contralateral lateral lemniscus and was distributed largely to its ventral nucleus. The contralateral central nucleus of the inferior colliculus was a major site of termination of ascending second order auditory fibers. The caudal tip of the ipsilateral ventral nucleus of the lateral lemniscus received abundant degeneration, but this diminished rostrally. The ipsilateral inferior colliculus contained a moderate amount of degeneration. A fair number of degenerated second order auditory fibers ascended in the contralateral brachium of the inferior colliculus and were distributed both to the principle and magnocellular divisions of the medial geniculate body. This pathway appears to represent a phylogenetic advance in the brain of the great ape.

The origins, course and distribution of the dorsal and intermediate acoustic striae in the rhesus monkey.

AbstractAttempts were made to produce discrete stereotactic lesions in the dorsal cochlear nucleus (Dc) in a series of rhesus monkeys. Anterograde degeneration was studied using Nauta and Fink‐Heimer techniques. Following lesions confined to the caudal pole of Dc, a few degenerated fibers projected to the contralateral lateral superior olivary nucleus via the dorsal acoustic stria. Degeneration was distributed to the contralateral ventral and to a lesser extent dorsal nuclei of the lateral lemniscus. The contralateral central nucleus of the inferior colliculus was a major site of termination of second order auditory fibers; a small number of degenerated fibers ascended to this nucleus ipsilaterally, and a few crossed in its commissure. A few degenerated fibers also were present in the contralateral magnocellular division of the medial geniculate body.When Dc was damaged at levels which overlie the posteroventral cochlear nucleus (Pv), or after lesions involving both subdivisions, degeneration was seen in the intermediate as well as in the dorsal acoustic stria. Fibers arising in Pv coursed through Dc and were interrupted by lesions in the latter. In addition to degeneration described above, fibers projected to the ipsilateral lateral superior olivary nucleus, contralateral medial trapezoid nucleus, and bilaterally to dorsal retroolivary and to preolivary cell groups. No degeneration was seen in the trapezoid body. Lesions involving the posteroventral and anteroventral (Av) cochlear nuclei produced degeneration in the intermediate acoustic stria and in the trapezoid body, but not in the dorsal stria. Degeneration in the intermediate acoustic stria was similar to that present after lesions affecting Dc and Pv. Thus, since Av does not project crossed fibers to this structure, these findings indicate that Pv sends some fibers to the contralateral as well as ipsilateral lateral superior olivary nucleus.

The neuropil of the periglomerular region of the olfactory bulb.

ABSTRACT The periglomerular region of the olfactory bulb, apart from containing the somata and stem dendrites of the cells contributing to the glomeruli, is the sole region of distribution of the periglomerular cell thin dendrites and the short-axon cell dendrites. It is also the major site of termination of all axons to the glomerular layer except the olfactory axons — i.e. tufted cell collaterals, periglomerular cell and short-axon cell axons and centrifugal fibres. Its characteristic neuropil has been studied with the electron microscope to define the cells of origin of the types of neuronal process and their synaptic relationships. Three types of axon terminals have been found: those with spherical, large flattened and small flattened vesicles, which are deduced to derive from tufted cell collaterals or centrifugal fibres, periglomerular cell and short axon-cell axons respectively; those with spherical vesicles are consistently associated with asymmetrical membrane thickenings and those with either type of flattened vesicles with symmetrical thickenings. The thin periglomerular cell dendrites are very irregular and often have a somewhat dense cytoplasm, rich in ribosomes; they may become extended into very attenuated glia-like sheets that surround the mitral or tufted cell stem dendrites, from which they may receive synaptic contacts. Such dendrites also receive some synapses from all 3 types of axon in the periglomerular region. The short-axon cell dendrites are thick and varicose and show no sign of synaptic specialization oriented from them; they have few spines but receive many asymmetrical-type synapses on their shafts. Both axon terminal types synapsing with symmetrical thickenings are also found on the shafts. The evidence obtained from the study of normal material is summarized and the various cellular roles considered. In the light of observations on the olfactory bulb, it is suggested that dendrites may be divided into 2 major classes: those that only receive synapses (Class A) and those that make synaptic contacts as well as receiving them (Class B). Further comparisons with the deep layers of the bulb and physiological implications are discussed.