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131.
Two complementary DNA clones pRLγ-2 and pRLγ-3 of different rat lens γ-crystallin messenger RNAs have been used to identify γ-crystallin gene sequences in rat genomic DNA. Subsequently, the DNA present in the 18,000 to 20,000 bases region of the EcoRI digest, giving rise to a strong doublet hybridization signal, was cloned in λ phage Charon-4A. One of the clones, λRCHγ-3, carrying an insert of 17,500 bases has been characterized in detail. From analysis at the restriction enzyme level with 5′-, “middle” and 3′-specific subprobes of pRLγ-3 it could be deduced that λRCHγ-3 contains only one γ-crystallin gene. The coding sequences of this gene are interrupted by intronic DNA. The primary structure of this gene and its flanking regions have been established by sequencing the relevant regions of a subclone of λRCHγ-3, designated pRCHγ-3.1. The sequence data show that the γ-crystallin gene extends over 2700 bases of rat genomic DNA. The gene is split by two introns, one of 87 base-pairs after the third translation codon and a large one of 1880 base-pairs after codon 84. The mosaic structure of the gene is strictly co-linear with the structure of the γ-crystallin polypeptide in that the large intron is positioned in a region which specifies the so-called “connecting peptide” and which links the two highly symmetrical and homologous protein domains. Although expected from the cDNA and protein sequence no introns were observed between the coding regions in the DNA specifying the two homologous folding motifs present in each protein domain. The relevance of this phenomenon in terms of the evolution of the mature γ-crystallin gene is discussed.  相似文献   
132.
The 1H nuclear magnetic resonance (nmr) spectra of complexes of soybean ferric leghemoglobin with 3-substituted pyridines and 5-substituted nicotinic acids have been recorded in order to determine the influence of axial ligands on heme electronic structure. The hyperfine shifted resonances of the heme group were assigned by analogy to previous assignments for the pyridine and nicotinic acid complexes of leghemoglobin. The spectra are characteristic of predominantly low-spin ferric heme complexes. For the pyridine complexes, the rate of ligand exchange was found to increase with decreasing ligand pKA. For many of the complexes, optical and nmr spectra reveal the presence of an equilibrium mixture of high- and low-spin states of the iron atom. The percentage of high-spin component increases with decreasing ligand pKA Smaller hyperfine shifts are noted for leghemoglobin complexes with ligands capable of weak ligand → metal π bonding. The pattern of hyperfine shifted resonances is similar for all complexes studied and indicates that the overall heme electronic structure is dominated by the bonding to the proximal histidine.  相似文献   
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Summary The role of microtubules in tracheary element formation in cultured stem segments ofColeus has been investigated through the use of the antimicrotubule drug, colchicine. Colchicine treatment of the cultured stem segments produced a dual effect on xylem differentiation. If applied at the time of stem segment isolation or shortly thereafter, wound vessel member formation is almost completely blocked. However, if colchicine is applied after the third day of culture, it does not inhibit differentiation, but instead large numbers of xylem elements are formed which have highly deformed secondary walls. Both effects are related to colchicine's specific affinity for microtubules. In the first case it is shown that colchicine blocks mitosis, presumably by destroying the spindle apparatus, and thus inhibits divisions which are prerequisite for the initiation of xylem differentiation. While, if colchicine is applied after the necessary preparative divisions have taken place, it destroys specifically the cortical microtubules associated with the developing bands of secondary wall, thus causing aberrant wall deposition.Light and electron microscopic analysis of drug-treated cells reveals that the secondary wall becomes smeared over the surface of the primary wall and does not retain the discrete banded pattern characteristic of secondary thickenings in untreated cells. Examination of colchicine-treated secondary walls in KMnO4 fixed material shows that in the absence of microtubules the cellulose microfibrils lose their normal parallel orientation and are deposited in swirls and curved configurations, and often lie at sharp angles to the axis of the secondary wall band. Microtubules, thus, appear to play a major role in defining the pattern of secondary wall deposition and in directing the orientation of the cellulose microfibrils of the wall. Factors in addition to microtubules also act in controlling the secondary wall pattern, since we observe that even in the absence of microtubules secondary thickenings of two adjacent xylem elements are deposited directly opposite one another across the common primary wall.  相似文献   
136.
New observations on the bioluminescence of echinoderms   总被引:3,自引:0,他引:3  
Peter J.  Herring 《Journal of Zoology》1974,172(3):401-418
Bioluminescence has been observed in two species of Ophiuroidea, six species of Asteroidea, nine species of Holothuroidea and two species of Crinoidea. The luminescent responses are described and the intensities and time courses of the responses of some species are given.
Histological investigations have indicated possible luminous cells in the Holothuroidea, but the results in other classes remain equivocal. Preliminary analyses of the chemistry of bioluminescence indicate that a similar system may be responsible for bioluminescence in all four classes, although the visible manifestations of the system vary considerably.  相似文献   
137.
The transport of uridine, hypoxanthine, and choline in cultures of Novikoff rat hepatoma cells is competitively inhibited by colcemid with apparent Ki values of 135, 60, and 250 μM respectively, whereas the transport of 2-deoxy-D-glucose is not affected. Ethanol at high concentrations inhibits the transport of all four substrates in an apparent competitive manner.  相似文献   
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