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51.
Specific cleavage of LexA repressor plays a crucial role in the SOS response of Escherichia coli. In vivo, cleavage requires an activated form of RecA protein. However, previous work has shown that the mechanism of cleavage is unusual, in that the chemistry of cleavage is probably carried out by residues in the repressor, and not those in RecA; RecA appears to facilitate this reaction, acting as a coprotease. We recently described a new type of lexA mutation, a class termed lexA (IndS) and here called IndS, that confers an increased rate of in vivo cleavage. Here, we have characterized the in vitro cleavage of these IndS mutant proteins, and of several double mutant proteins containing an IndS mutation and one of several mutations, termed Ind-, that decrease the rate of cleavage. We found, first, that the autodigestion reaction for the IndS mutant proteins had a higher maximum rate and a lower apparent pKa than wild-type LexA. Second, the IndS mutations had little or no effect on the rate of RecA-mediated cleavage, measured at low protein concentrations, implying that the value of Kcat/Km was unaffected. Third, the rate of autodigestion for the double-mutant proteins, relative to wild-type, was about that rate predicted from the product of the effects of the two single mutations. Finally, by contrast, these proteins displayed the same rate of RecA-mediated cleavage as did the single Ind- mutant protein. We interpret these data to mean that the IndS mutations mimic to some extent the effect of RecA on cleavage, perhaps by favoring a conformational change in LexA. We present and analyze a model that embodies these conclusions. 相似文献
52.
Roland Prinzinger 《Journal of Ornithology》1990,131(1):47-61
53.
Summary The tight junctions along the medullary collecting duct in the kidneys of the rat and the rabbit were studied with freeze-fracture electron microscopy and quantitated according to the number of strands and the apico-basal depth (nm) of the junctions.The most elaborate tight junctions were found in the inner stripe of the outer medulla; rat: 10.6±0.8 strands and 205±24nm; rabbit: 11.6±2.4 strands and 291±55 nm.The elaboration of the tight junctions decreased continuously towards the papillary tip. Inner zone I; rat: 9.3±2.6 strands and 186±38nm, rabbit: 9.5±2.3 strands and 247±59nm. Inner zone II; rat: 7.1±2.2 strands and 129±32nm, rabbit: 8.5±1.4 strands and 199±26nm. Inner zone III; rat: 6.0±1.6 strands and 111 + 19 nm, rabbit: 7.0±1.5 strands and 183±43 nm. In the inner zone III comprising the papillary tip tight junctions with only 1–3 strands were not infrequently seen. Preliminary findings in the kidney of the golden hamster indicate a similar decline of junctional tightness along the collecting duct.These morphological observations suggest that the permeability of the paracellular pathway of the medullary collecting duct increases towards the tip of the papilla, especially in the rat. The functional implications for the medullary recycling of urea and electrolytes, and for the urinary concentrating mechanism are discussed.In addition, the tight junctions of the papillary epithelium are described. 相似文献
54.
Roland Kühne 《Journal of insect physiology》1982,28(2):155-163
The response characteristics of the vibration receptors in the legs of the migratory locust, Locusta migratoria, and the tettigoniid Decticus verrucivorus were investigated electro-physiologically by single cell recordings. The legs were stimulated by sinusoidal vibrations. There are four types of vibration receptor in each leg of Locusta and Decticus, which can be classified physiologically. One type—most probably campaniform sensilla—shows a phase-locked response to vibrations from 30 to 200 Hz, its threshold reflecting the displacement. A second type shows similar responses in the same frequency range, but its reactions depend on the stimulus acceleration. The receptor cells of the subgenual organ are very sensitive to vibration from 30 to at least 5000 Hz, and their responses depend on acceleration. There are two types of subgenual receptors, one of which shows a clear maximum of sensitivity between 200 and 1000 Hz, with a threshold below 0.01 m/sec?2 acceleration. Subgenual receptors with different thresholds and different characteristic frequencies occur in each leg. The receptors of each leg pair have quite similar mean sensitivities and characteristic frequencies. However, in the front legs of tettigoniids the more sensitive subgenual receptors and an additional receptor type also respond to low-frequency airborne sound up to 10 kHz. 相似文献
55.
Summary Quantitative aspects of tight junction morphology were systematically studied in the cortical and outer medullary segments of the distal urinary tubules of rat, hamster, rabbit, cat, dog and the primitve primate Tupaia belangeri.Only minor differences in junctional architecture were found between straight and convoluted portions of the distal tubule. In contrast, the collecting duct in cortex and outer medulla, in all species, exhibits the most elaborate tight junctions observed along the uriniferous tubule.The present and previous findings from this laboratory indicate that increasing tightness of the junctional complexes is apparent along the course of the nephron in all species studied.The proposed relationship between quantitative aspects of the zonula occludens and presently available values for transepithelial electrical resistance was re-examined for the renal tubules. It was found that for the mammalian kidney a satisfactory correlation exists between the tight junction morphology and presently known functional parameters. This relationship is the more evident the more additional dimensional characteristics of the intercellular clefts are taken into consideration.It may therefore be concluded that, at least for the mammalian kidney, the assumption of differences in the molecular organization of the tight junctions is not needed to explain so far unresolved discrepancies between tubular morphology and function.Parts of these findings were presented at the 72nd Meeting of the Anatomical Society, Aachen; April 1977 (see Verh. Anat. Ges. 72:229–234 [1978])Supported by the Deutsche Forschungsgemeinschaft 相似文献
56.
Robert O. Hussa Roland A. Pattillo 《In vitro cellular & developmental biology. Plant》1980,16(7):585-590
Summary The BeWo line of trophoblastic cells, maintained in continuous culture since 1966, was employed to investigate the phenomenon
of gonadotropin α-subunit predominance that exists in several cell lines. The secretion of complete human chorionic gonadotropin
(hCG) relative to α-subunit was compared in several different BeWo sublines, all of which were derived from BeWo stock roller
tube colonies. In all of the BeWo sublines, secretion of hCG originally exceeded secretion of α-subunit. With time in culture,
however, there was a marked decline in production of hCG/hCGβ, but not in α-subunit. Thus it appears that the production of
hCGβ by BeWo choriocarcinoma cells is more labile than the production of the α-subunit. 相似文献
57.
Beate Ackermann-Schmidt Roland Süssmuth Franz Lingens 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,699(2)
The mutagen 1,1′-hexamethylene-bis[(5-p-chlorophenyl)-biguanide] reacts at 37°C with guanosine and guanine to yield xanthosine or xanthine and oxidizes cysteine to cystine. After treatment of a guanosine-labelled DNA sample from Escherichia coli with the mutagen xanthine could be detected as a reaction product. At a slow rate the mutagen is hydrolysed spontaneously yielding urea, 1,6-hexanediol and 4-chloroaniline. The reaction mechanisms both of the hydrolysis and of the reaction with cysteine and guanosine are discussed. 相似文献
58.
59.
R. Taugner U. Boll P. Zahn Prof. Dr. W. G. Forssmann III 《Cell and tissue research》1976,172(4):431-446
Summary The intercellular connections between the epithelial cells of Bowman's capsule were investigated. It could be demonstrated that typical zonulae occludentes (tight junctions) are present in the species (rat, hamster, and Tupaia) studied. Freeze-fracturing shows a network of anastomizing strands; some species variations are described. In the rat two strands are common. In the golden hamster mostly two to four and occasionally five strands occur. In Tupaia regularly three tight junction strands are found and also gap junctions associated with the zonulae occludentes. In thin sections the goniometric analysis confirms the freeze-fracturing results and reveals attachment zones of macular shape, which are classified as intermediate junctions and desmosomes. The functional role of these cell junctions observed in the epithelium of Bowman's capsule is discussed. 相似文献
60.