Morphogenesis in Stem-callus Cultures of Convolvulus arvensis L

Convolvulus arvensis stem explants form callus when inoculatedon to 0.5 mgm/1 2, 4-D, 15 per cent coconut milk, and on to1 mgm/1 kinetin, 15 per cent coconut milk. In the latter casenumerous shoots are formed on the callus. Callus formed on explantson 0.05 mgm/1 2, 4-D, 15 per cent coconut milk formed shootswith in-creasing vigour when transferred to a medium (1) inwhich the 2, 4–D concentration is lowered by one-tenth,(2) which contains no auxin but kinetin at 1 mgm/1, (3) whichcontains kinetin at 1 mgm/1 and 15 per cent coconut milk. OnNAA/ kinetin media shoots may form in which the intenode developmentis suppressed, giving compressed shoots. The capacity of callusto form shoots may be retained or lost through repeated sub-culture.Roots are formed erratically. The results are discussed, particularlywith regard to the loss of morphogenetic potential and whetherthis is reversible.     

The Neisseria gonorrhoeae gene aniA encodes an inducible nitrite reductase

The yeast nuclear mutation mgm104-1, which leads to slow growth on glucose medium and temperature-sensitive (ts) loss of mitochondrial DNA (mtDNA), has been identified by screening a collection of temperature-sensitive mutants on glycerol medium. A nuclear gene was isolated from a genomic DNA library by complementation of the mgm104-1 allele and was found to be identical to TTS1, which encodes the cytoplasmic tyrosyl-tRNA synthetase required for cytoplasmic protein synthesis. A gene disruption in a diploid strain demonstrated that the TTS1 gene is essential for cell viability. The lack of mutations in TTS1 in the mgm104-1 mutant indicates that TTS1 and MGM104 are different genes. The ability to rescue the mgm104-1 phenotype with a single additional copy of TTS1 suggests that TTS1 has an additional function that is directly or indirectly involved in the maintenance of the mitochondrial genome.     

CARBON DIOXIDE FROM THE NERVE CORD OF THE LOBSTER

1. The nerve cord of the lobster (Homarus americanus Milne-Edwards) is very delicate and can be used as a living preparation for only a few hours after its removal from the animal. 2. During the first hour or so after removal it discharges CO₂ at a steadily decreasing rate beginning at about 0.20 mg. CO₂ per gram of cord per minute and ending at about 0.07 mg. 3. This discharge exhibits a steady decrease in rate and is not divisible into a period of gush and a period of uniform outflow as with the lateral-line nerve of the dogfish. It terminates in a very few hours with the complete death of the cord. 4. Both handling and cutting the cord temporarily increase the rate of CO₂ output. 5. The stimulated cord discharges CO₂ at a rate about 26 per cent higher than that of the quiescent cord, an increase of about 1.6 times that of the increase observed in the lateral-line nerve of the dogfish under similar circumstances.     

THE EXCRETION OF CARBON DIOXIDE BY FROG NERVE

1. Quiescent sciatic nerve of the frog discharges CO₂ at the average rate of 0.00876 mg. CO₂ per gram of nerve per minute. 2. Sciatic nerve steeped one minute in boiling water discharges CO₂ at first at a low rate and after an hour and a half not at all. 3. Degenerated sciatic nerve discharges CO₂ at a slightly higher rate than normal living nerve does. 4. Connective tissue from the frog discharges CO₂ at an average rate of 0.0097 mg. per gram of tissue per minute. 5. Assuming that a nerve is composed of from one-half to one-quarter connective tissue the CO₂ output from its strictly nervous components is estimated to be at a rate of 0.008 mg. CO₂ per gram of nerve per minute. 6. Stimulated sciatic nerve increases the rate of its CO₂ output over quiescent nerve by about 14 per cent. When this number is corrected for strictly nervous tissue the rate is about 16 per cent. 7. The increased rate of CO₂ production noted on stimulation in normal sciatic nerves was not observed when they were boiled, blocked, or degenerated. It was also not observed with stimulated strands of connective tissue.     

FURTHER OBSERVATIONS ON THE FINE STRUCTURE OF THE PARIETAL EYE OF LIZARDS

An electron microscopical study of the third eye of the Western Fence Lizard, Sceloporus occidentalis, fixed with 1 per cent osmium tetroxide, pH 7.4–7.6, for 16 to 20 hours at 0°C., revealed the following new facts. The fibrillar system of the retinal photoreceptor consists of nine double fibrils enclosed in a sheath. Pigment cells and lens cells possess similar systems. Two short cylindrical centrioles are associated with the fibrillar apparatus: one, from which striated rootlets extend inward, lies at the base of the fibrils, with the other at an oblique angle to the axis of the system. A Golgi complex, whorls of endoplasmic reticulum, lipid (?) droplets, and other organelles and inclusions in the photoreceptors are described. An axon leads from the base of the photoreceptor into the nervous layer of the retina which consists of many nerve fibers and large ganglion cells. Although the pattern of neural connections has not yet been determined, some synapses were found. The parietal nerve consists of about 250 non-medullated fibers. The capsule of the eye usually has a layer of iridocytes, which contain rows of guanine (?) rods. A few parietal eyes of the Granite Night Lizard, Xantusia henshawi, were also examined. Large lipid (?) droplets occur in the bases of their receptoral processes.     

ACETYLCHOLINE CONTENT OF THE SCHWANN CELL AND AXON IN THE GIANT NERVE FIBRE OF THE SQUID

Abstract— Acetylcholine and choline were identified and their concentrations measured, by means of gas chromatography/mass spectrometry, in extracts obtained from nerve fibers of the hindmost stellar nerve of the squid Sepioteuthis sepioidea. These compounds were quantitated in samples of stellar nerve devoid of giant fiber, intact giant nerve fiber, extruded axoplasm, and axoplasm-free giant nerve fiber sheaths. In 11 samples of stellar nerve devoid of giant fiber, weighing an average of 20.8 ± 2.3 mg ( s.e.m. ), 756 ± 91 pmol ACh and 8.65 ± 0.62 nmol of choline were found. The total ACh content of the largest fibre in this group (10 μ m in diameter), for a 5 cm length of nerve, is in the order of 0.16 pmol. The average wet weights of a single giant nerve fiber (270-420 μ m in diameter) and its separate components ( s.e.m .; in mg; number of fibers in parentheses) were: intact fiber, 4.58 ± 0.19 (25); extruded axoplasm, 3.38 ± 0.13 (20); sheaths, 1.21 ± 0.11 (16). The average ACh content per unit weight of sample was about 2-3 times higher in the sheaths (5-13 pmol-mg⁻¹) than in the axoplasm (2-4 pmol mg⁻¹), whereas the ACh concentrations estimated per unit volume of cellular water were about 40 times higher in the Schwann cell (107-222 μ m ) than in the axon (2-5 μ m ). These experimental findings establish the presence of ACh in the giant nerve fiber of S. sepioidea. They also indicate the Schwann cells themselves as the main source for the release of ACh, responsible for their long-lasting hyperpolarizations following the conduction of nerve impulse trains by the axon.     

Inhibition in the eye of Limulus

In the compound lateral eye of Limulus each ommatidium functions as a single receptor unit in the discharge of impulses in the optic nerve. Impulses originate in the eccentric cell of each ommatidium and are conducted in its axon, which runs without interruption through an extensive plexus of nerve fibers to become a fiber of the optic nerve. The plexus makes interconnections among the ommatidia, but its exact organization is not understood. The ability of an ommatidium to discharge impulses in the axon of its eccentric cell is reduced by illumination of other ommatidia in its neighborhood: the threshold to light is raised, the number of impulses discharged in response to a suprathreshold flash of light is diminished, and the frequency with which impulses are discharged during steady illumination is decreased. Also, the activity that can be elicited under certain conditions when an ommatidium is in darkness can be inhibited similarly. There is no evidence for the spread of excitatory influences in the eye of Limulus. The inhibitory influence exerted upon an ommatidium that is discharging impulses at a steady rate begins, shortly after the onset of the illumination on neighboring ommatidia, with a sudden deep minimum in the frequency of discharge. After partial recovery, the frequency is maintained at a depressed level until the illumination on the neighboring receptors is turned off, following which there is prompt, though not instantaneous recovery to the original frequency. The inhibition is exerted directly upon the sensitive structure within the ommatidium: it has been observed when the impulses were recorded by a microelectrode thrust into an ommatidium, as well as when they were recorded more proximally in single fibers dissected from the optic nerve. Receptor units of the eye often inhibit one another mutually. This has been observed by recording the activity of two optic nerve fibers simultaneously. The mediation of the inhibitory influence appears to depend upon the integrity of nervous interconnections in the plexus: cutting the lateral connections to an ommatidium abolishes the inhibition exerted upon it. The nature of the influence that is mediated by the plexus and the mechanism whereby it exerts its inhibitory action on the receptor units are not known. The depression of the frequency of the discharge of nerve impulses from an ommatidium increases approximately linearly with the logarithm of the intensity of illumination on receptors in its vicinity. Inhibition of the discharge from an ommatidium is greater the larger the area of the eye illuminated in its vicinity. However, equal increments of area become less effective as the total area is increased. The response of an ommatidium is most effectively inhibited by the illumination of ommatidia that are close to it; the effectiveness diminishes with increasing distance, but may extend for several millimeters. Illumination of a fixed region of the eye at constant intensity produces a depression of the frequency of discharge of impulses from a nearby ommatidium that is approximately constant, irrespective of the level of excitation of the ommatidium. The inhibitory interaction in the eye of Limulus is an integrative process that is important in determining the patterns of nervous activity in the visual system. It is analogous to the inhibitory component of the interaction that takes place in the vertebrate retina. Inhibitory interaction results in the exaggeration of differences in sensory activity from different regions of the eye illuminated at different intensities, thus enhancing visual contrast.     

CHANGES IN THE BASE COMPOSITION OF NUCLEAR RIBONUCLEIC ACID OF NEURONS DURING A SHORT PERIOD OF ENHANCED PROTEIN PRODUCTION

Nuclei from isolated nerve cells were sampled by microdissection. The content and composition of the nuclear RNA was studied and compared with that of the cytoplasmic RNA of Deiters' nerve cells of rabbits. Analyses were made of control nerve cells and of cells in which an enhanced RNA and protein production had been induced by chemical means, tricyano-amino-propene, for 60 minutes. The nuclear RNA content of the control nerve cells was 56 µµg, i.e. 3 per cent of the total RNA content of the nerve cell. The base ratios were: adenine 21.3, guanine 26.6, cytosine 30.8, uracil 21.3. Purine-pyrimidine analyses showed that the nuclear RNA differed significantly from the cytoplasmic RNA in having higher adenine and uracil values. The guanine and cytosine values were high, however, and the ratio G/C was 0.86 as compared with 1.16 for the cytoplasmic RNA. The composition of the nuclear RNA was interpreted as reflecting the extraordinarily strong development of the nucleolus in these neurons. During the 60 minutes of enhanced neuronal RNA production (+25 per cent) the guanine value increased and the uracil value decreased significantly in the nuclear RNA. In the cytoplasmic RNA the guanine value also increased although not so much as the nuclear guanine. The cytoplasmic cytosine value decreased. The result indicated that the production of the characteristic cytoplasmic RNA had been influenced by the change in the nuclear RNA     

THE FORMATION OF CHOLINE AND OF ACETYLCHOLTNE BY BRAIN IN VITRO

Abstract— Free choline and acetylcholine (ACh) in mouse or rat brain were assayed biologically. The subcellular distribution of ACh in brain slices that had been incubated in the presence of eserine was compared to that in control brain; during incubation, the ACh outside nerve endings increased four-fold, the ACh released from synaptosomes by osmotic shock doubled but the ACh bound firmly within nerve endings did not increase. The two nerve ending stores of ACh were labelled to similar specific radioactivities when slices were incubated with [³H]choline, but the specific radioactivity of the ACh formed was much lower than that of the added choline. Tissue incubated in the presence of eserine released choline and ACh into the medium and the tissue levels of both substances increased. Brain tissue exposed to Na⁺-free medium lost 84 per cent of its ACh and 66 per cent of its free choline; the amounts of both substances returned towards control values during subsequent incubation in a normal-Na⁺ medium (choline-free). Both the ACh outside nerve endings and the ACh associated with synaptosomes were depleted when tissue was incubated in Na⁺-free medium.     

THE GRANA OF STARCH-FREE CHLOROPLASTS OF NICOTIANA RUSTICA

The grana of chloroplasts of starch-free leaves of Nicotiana rustica are described in detail. Leaf sections were fixed in 2.5 per cent KMnO₄ and embedded in mixtures of butyl and ethyl methacrylate. Chain length of the polymer was modified by use of a transfer agent. The grana are composed of compartments consisting of electron-scattering partitions and electron-transparent loculi. Compartments are not open to the stroma so that the grana are distinct subplastid organelles. Adjacent grana are connected by an anastomosing fretwork system composed of flexuous channels bordered by electron-scattering membranes. Ten different kinds of granum margins are described. These marginal variations depend upon grana-fretwork connections and internal marginal connections between adjacent loculi. A study of serial sections suggests, at least in some plastids, the occurrence of a possible orderly spiral arrangement of compartment-fretwork connections. Adjacent grana may have common compartments. Grana may branch. Variations in shape may depend upon the angle the section bears to the axis of the cylinder. This should also influence the relative thickness and sharpness of the partitions. Since all shapes and variations in partition thickness and sharpness cannot be accounted for on the basis of the orientation of the grana, such variations probably occur naturally. Grana vary in size, ranging from those few which have a single partition to those having 50 or more compartments which extend completely through the width of a plastid. Relationships between grana of different sizes and between compartments and frets indicate the possibility of growth of grana from union or extension of compartments and formation of compartments from the union of frets.     

Adaptation of rhizome connections in drylands: increasing tolerance of clones to wind erosion

Background and Aims

Wind erosion is a severe stress for plants in drylands, but the mechanisms by which plants withstand erosion remain largely unknown. Here, the hypothesis is tested that maintaining rhizome connections helps plants to tolerate erosion.

Methods

Five transects were established across an inland dune in Inner Mongolia, China, and measurements were made of leaf number, biomass per ramet and rhizome depth of Psammochloa villosa in 45 plots. In 40 × 40 cm plots of P. villosa on another dune, the top 15 or 30 cm of sand was removed for 1·5 or 3 months to simulate short- and long-term moderate and severe erosion, respectively, with untreated plots as controls, and the rhizomes at the edges of half of the plots were severed to mimic loss of rhizome connections.

Key Results

Leaf number and biomass per ramet showed quadric relationships with rhizome depth; when rhizomes were exposed to the air, the associated ramets either died or became very weak. Ramet number, leaf number and biomass per plot decreased with increasing erosion severity. Rhizome connections did not affect these traits under control or short-term erosion, but increased them under long-term erosion.

Conclusions

Rhizome connections alleviated the negative effects of erosion on P. villosa, very likely because the erosion-stressed ramets received water and/or photosynthates translocated from those connected ramets that were not subject to erosion. This study provides the first evidence that maintaining rhizome connections helps plants to tolerate erosion in drylands.Key words: Clonal integration, inland-dune grass, Psammochloa villosa, resource sharing, rhizome severing, wind erosion     

THE LOCUS OF THE ACTION OF VERATRIN

1. In Mnemiopsis veratrin shows two stages of veratrin poisoning. First, inhibition of the beats of the plates which disappears on cutting them away either singly or in small groups. Second, after half an hour mechanical stimulation excites the beat of the plates in the intact veratrinized animal. It is concluded that veratrin acts on nervous tissue and not on the substance of the swimming plates. 2. In Lumbricus, veratrin acts on the ventral nerve cord alone, and not on the muscles and peripheral nerves. 3. In Musca, veratrin first causes opisthotonos, then spasms and extreme flexion of the legs. Decapitation causes these effects to disappear hence veratrin acts on the cerebral ganglia of the fly. 4. Veratrin applied to the sciatic nerve of the frog causes, after a latent period of 20 minutes, irregular contractions of the gastrocnemius which persist for an hour or more. Veratrin is thus a neurophil alkaloid of the first class as well as second and in this way resembles tetraethyl ammonium chloride. 5. If the end of a sciatic nerve is dipped into veratrin solution, then direct stimulation of the gastrocnemius muscle results in contraction with delayed relaxation, although the muscle itself is not subject to the action of veratrin. 6. By means of preparations of the sartorius muscle of the frog it is shown that veratrin acts not on the muscle cells directly but on the nerve fibers. Hence veratrin produces the characteristic muscle curve showing delayed relaxation by its action on the nervous elements.     

The mode of regulation of the corpus allatum activity during starvation in adult females of the Colorado potato beetle, Leptinotarsa decemlineata (Say)

When two-day-old female Leptinotarsa decemlineata were starved, their corpus allatum activity, as measured by the radiochemical in vitro assay, was significantly reduced after 24 hr. Such a reduction was not observed when the nerve connections between the central nervous system and the retrocerebral complex were severed and the beetles starved up to 5 days. In some experiments, the rate of juvenile hormone biosynthesis in vitro, was substantiated by measurement of the juvenile hormone titre in the haemolymph by physico-chemical methods. It is concluded that intact nervous connections between the central nervous system and the corpora allata are essential for restraining the juvenile hormone biosynthesis during the initial stages of starvation.Corpora allata from 1-day starved insects were considerably stimulated in vitro by farnesenic acid indicating that juvenile hormone synthesis is controlled enzymatically at a stage prior to the final two steps in the pathway. However, on day 5 of starvation, rate-limitation may occur after formation of this intermediate, since farnesenic acid stimulation was much less at this time.Corpora allata of adult females newly emerged from the soil were activated within 4 hr regardless of feeding.     

STUDIES ON THE CROSS-STRIATION OF THE INDIRECT FLIGHT MYOFIBRILS OF THE BLOWFLY CALLIPHORA

1. The cross-striation in the indirect flight myofibrils of Calliphora has been studied by phase contrast and polarised light microscopy. The band pattern at rest-length has been determined in flies killed in osmium tetroxide vapour while their wings remained in the resting position. All other observations have been made on unfixed fibrils. Although length changes in situ are probably very slight (about 2 per cent), isolated fibrils, by treatment with crude muscle extract or with ATP, can be induced to elongate to 104 per cent rest-length, or to shorten by 8 per cent but no more. Over the range 98 to 104 per cent rest-length, experimentally induced length changes are reversible. The fibrils can also be stretched beyond 104 per cent rest-length, but the process is irreversible. During the course of glycerol extraction the fibrils elongate to 104 per cent rest-length. 2. The changes in band pattern observed over the range 104 to 92 per cent rest-length are qualitatively the same as the changes observed over a wider range (about 130 to 40 per cent rest-length) in the skeletal myofibrils of rabbits. The earlier stages of shortening appear to be effected by retraction of the I bands into the A bands where they fill up the H zones. No evidence has been found that any changes in band pattern are due to a migration of the A substance. 3. Two components of the sarcomere can be extracted from it and a third component remains behind. These three components, which have also been demonstrated in skeletal myofibrils of the rabbit, where they behave in the same way, are: (a) the A substance which does not change its position as the fibril changes its length, and which can be extracted by the same procedures as remove myosin (shown elsewhere to be the A substance) from rabbit fibrils; (b) a material which extends from the Z lines to the borders of the H zone and which moves inwards during contraction and outwards during elongation; it can capture rabbit myosin from solution and form with it a contractile system, and it is thought to be actin; (c) a "backbone" or stroma bearing Z and M lines. 4. Since all these features of the cross-striation are the same in the insect fibrils as in rabbit fibrils, it is considered very probable that the sarcomere is similarly organised in both types of muscle and contracts by essentially the same mechanism.     

FAST AXONAL TRANSPORT IN VITRO IN THE SCIATIC SYSTEM OF THE FROG

Abstract— An in vitro system from the frog has been used to study fast axonal protein transport. The preparation, which was incubated in a specially made chamber, consisted of the gastrocnemius muscle, the sciatic nerve, the dorsal ganglia and part of the spinal cord. The parts were separated from each other by silicone grease barriers, which made it possible to follow the migration of labelled proteins from the spinal cord and ganglia, along the sciatic nerve, towards the muscle. About 80 per cent of transported proteins in the sciatic nerve originated from the dorsal spinal ganglia and moved antidromically at a rate of 60–90 mm per day at 18°C. The rapidly transported proteins were 90 per cent particulate and mainly associated with structures sedimenting in the microsomal fraction.
The effects of cyclohexirnide showed that the synthesis of rapidly moving proteins and their transport were separate processes. A low concentration of colchicine inhibited the transport when it was present in the medium surrounding the ganglia, but had no effect even at a higher concentration, when it was added to the nerve compartment. The presence of vinblastine at a low concentration in either of the two compartments completely arrested the protein transport. Likewise N-ethylmaleimide or p-chloromercuribenzene sulphonic acid in the nerve medium effectively blocked the fast transport. Results from experiments performed to test the possibility of disto-proximal flow and of transfer of proteins from the muscle to the nerve are discussed.     

THE RESPIRATORY QUOTIENT OF FROG NERVE DURING STIMULATION

1. By means of a differential volumeter the increased oxygen consumption and the increased carbon dioxide output of frog nerve during and after stimulation have been observed. 2. Measurements of the R.Q. of nerve by this method are complicated by the retention of carbon dioxide. Attempts were made to avoid this (a) by studying the nerves at high CO₂ tensions to make the retention small and (b) by calculating the amount of CO₂ retained from the carbon dioxide dissociation curve of nerve and applying this value as a correction. 3. The results of both those methods when averaged together give an R.Q. of the excess metabolism of 1.19 and an R.Q. of the resting nerve of 0.97. 4. Observations on the time course of the gas exchange during stimulation indicate a delay in the appearance of the extra carbon dioxide output relative to the oxygen intake. 5. Very similar time curves can be calculated from the diffusion coefficients and the solubilities of the oxygen and the carbon dioxide.     

SOME OBSERVATIONS ON THE FINE STRUCTURE OF THE LATERAL LINE ORGAN OF THE JAPANESE SEA EEL LYNCOZYMBA NYSTROMI

The fine structure of the lateral line organ of the Japanese sea eel Lyncozymba nystromi has been studied with the electron microscope. The sensory epithelium of the lateral line organ consists of a cluster of two major types of cells, the sensory hair cells and the supporting cells. The sensory cell is a slender element with a flat upper surface provided with sensory hairs, Two different types of synapses are distinguished on the basal surface of the receptor cell. The first type is an ending without vesicles and the second type is an ending with many vesicles. These are presumed to correspond to the afferent and the efferent innervations of the lateral line organ. The fine structure of the supporting cells and the morphological relationship between the supporting cells and the receptor cells were observed. The possible functions of the supporting cells are as follows: (a) mechanical and metabolic support for the receptor cell; (b) isolation of the individual receptor cell; (c) mucous secretion and probably cupula formation; (d) glial function for the intraepithelial nerve fibers. Both myelinated and unmyelinated fibers were found in the lateral line nerve. The mode of penetration of these fibers into the epithelium was observed.     

ISOLATION OF NERVE ENDINGS FROM THE POSTERIOR PITUITARY GLAND : Electron Microscopy of Fractions Obtained by Centrifugation

Bovine posterior pituitary glands were homogenized in 10 per cent sucrose and fractionated by differential centrifugation. The following centrifugation procedure resulted in the most satisfactory separation: 1000 g for 15 minutes—nuclei, connective tissue, basement membranes with associated endothelium, giant nerve endings, and whole pituicytes; 4200 g for 15 minutes—free nerve endings, including Herring bodies; 17,000 g for 15 minutes—mitochondria; 68,000 g for 15 minutes—neurosecretory granules. Electron microscopic examination was carried out on whole tissue and on the isolated fractions. Isolated nerve endings were examined also by negative staining techniques. Isolated nerve endings retain an apparently normal complement of mitochondria, neurosecretory granules, and microvesicles ("synaptic" vesicles). The free nerve endings closely resemble those observed in sections of intact posterior pituitary tissue. Free microvesicles were not observed in any of the fractions isolated and apparently sediment at centrifugal forces higher than those employed in this study.     

The yeast dynamin-like protein, Mgm1p, functions on the mitochondrial outer membrane to mediate mitochondrial inheritance

The mdm17 mutation causes temperature-dependent defects in mitochondrial inheritance, mitochondrial morphology, and the maintenance of mitochondrial DNA in the yeast Saccharomyces cerevisiae. Defects in mitochondrial transmission to daughter buds and changes in mitochondrial morphology were apparent within 30 min after shifting cells to 37 degrees C, while loss of the mitochondrial genome occurred after 4-24 h at the elevated temperature. The mdm17 lesion mapped to MGM1, a gene encoding a dynamin-like GTPase previously implicated in mitochondrial genome maintenance, and the cloned MGM1 gene complements all of the mdm17 mutant phenotypes. Cells with an mgm1-null mutation displayed aberrant mitochondrial inheritance and morphology. A version of mgm1 mutated in a conserved residue in the putative GTP-binding site was unable to complement any of the mutant defects. It also caused aberrant mitochondrial distribution and morphology when expressed at high levels in cells that also contained a wild-type copy of the gene. Mgm1p was localized to the mitochondrial outer membrane and fractionated as a component of a high molecular weight complex. These results indicate that Mgm1p is a mitochondrial inheritance and morphology component that functions on the mitochondrial surface.     

Nonacceptance of innervation by innervated neonatal rat muscle

Denervated adult muscle accepts innervation and has high levels of extrajunctional acetylcholine (ACh) receptor, compared to innervated adult muscle. If the high receptor density or any externally oriented part of the receptor molecule permitted or triggered functional synaptogenesis, then innervated neonatal muscle, with its known high extrajunctional sensitivity, should also accept extra synapses from implanted motor nerves. This prediction was tested by implanting the common peroneal nerve into innervated lateral gastrocnemius muscle in 25 neonatal rats and studying the innervation achieved 1–8 weeks later. With one exception, zero or negligible twitch tensions were obtained when the implanted nerve was stimulated. Intracellular recording in two cases showed no evidence of subthresholdevoked potentials in surface muscle fibers. In contrast, when the original motor nerve was cut at the time of common peroneal nerve implantation, reinnervation occurred as soon as 4 days later, and substantial indirect twitches (most observed qualitatively) were invariably found 6–7 days after operation. Four to eight weeks after nerve implantation into denervated muscle, substantial twitch tensions were obtained upon stimulation of the implanted nerve. α-Bungarotoxin binding to extrajunctional ACh receptors per unit surface area was similar in innervated neonatal and denervated adult muscle. Therefore, nonacceptance of additional functional innervation in neonatal muscle implies that a high average density of extrajunctional ACh receptor is not sufficient to permit or trigger functional neuromuscular junction formation.