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Oscar Felsenfeld 《Microbiological reviews》1965,29(1):46-74
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Direct sequencing of the mitochondrial displacement loop (D-loop) of shrews
(genus Sorex) for the region between the tRNA(Pro) and the conserved
sequence block-F revealed variable numbers of 79-bp tandem repeats. These
repeats were found in all 19 individuals sequenced, representing three
subspecies and one closely related species of the masked shrew group (Sorex
cinereus cinereus, S. c. miscix, S. c. acadicus, and S. haydeni) and an
outgroup, the pygmy shrew (S. hoyi). Each specimen also possessed an
adjacent 76-bp imperfect copy of the tandem repeats. One individual was
heteroplasmic for length variants consisting of five and seven copies of
the 79-bp tandem repeat. The sequence of the repeats is conducive to the
formation of secondary structure. A termination-associated sequence is
present in each of the repeats and in a unique sequence region 5' to the
tandem array as well. Mean genetic distance between the masked shrew taxa
and the pygmy shrew was calculated separately for the unique sequence
region, one of the tandem repeats, the imperfect repeat, and these three
regions combined. The unique sequence region evolved more rapidly than the
tandem repeats or the imperfect repeat. The small genetic distance between
pairs of tandem repeats within an individual is consistent with a model of
concerted evolution. Repeats are apparently duplicated and lost at a high
rate, which tends to homogenize the tandem array. The rate of D- loop
sequence divergence between the masked and pygmy shrews is estimated to be
15%-20%/Myr, the highest rate observed in D-loops of mammals. Rapid
sequence evolution in shrews may be due either to their high metabolic rate
and short generation time or to the presence of variable numbers of tandem
repeats.
相似文献
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大鼠胼胝体内神经肽Y免疫反应阳性纤维的发育 总被引:1,自引:0,他引:1
本实验用免疫组织化学ABC法研究了大鼠胼胝体内神经肽Y免疫反应阳性(NPY-IR)纤维的生后发育。结果发现,许多NPY-IR纤维在大鼠出生时便存在于胼胝体内。NPY-IR胼胝体纤维的密度在生后1周内继续逐渐增高,在第2周内达到最高峰。之后,NPY-IR胼胝体纤维的密度逐渐下降,至第3周末时接近成年时的水平,即仅有少量NPY-IR纤维存在于胼胝体内。这些结果提示在大鼠早期生后发育过程中许多NPY-IR胼胝体纤维是暂时性的,其作用可能与大脑皮质的机能发育有关。 相似文献
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Isolation and characterization of recombinant clones containing the chicken adult beta-globin gene. 总被引:5,自引:0,他引:5
We have isolated and characterized two independent clones containing the chicken adult beta-globin gene. Each clone contains a 6.2-kilobase-pair Eco RI restriction fragment of chicken erythrocyte DNA inserted into the vector, lambda gtWES . lambda B. The orientation of the inserted fragment is opposite in the two clones. Characterization of the clones by electron microscopic R-loop studies, by restriction enzyme mapping, and by filter hybridization shows that the adult beta-globin gene is interrupted by at least one small and one large intervening sequence. In addition to the complete adult beta-globin gene, at least part of a second beta-globin-like gene was identified about 2.7 kilobase pairs from the 3'-end of the adult gene. The two independent clones, while very similar, do differ at two Msp I restriction endonuclease sites in regions flanking the adult beta-globin gene. 相似文献
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Supercoiling energy and nucleosome formation: the role of the arginine-rich histone kernel. 总被引:35,自引:16,他引:19
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We have formed complexes of relaxed closed circular Col E1 DNA with various combinations of histones, and examined the effects of treating the complexes with nicking-closing enzyme. Germond et al (1) have shown that when a mixture of the four core histones of the nucleosome (HIA, H2B, H3 and H4) is used in such an experiment, the subsequently isolated DNA is supercoiled. We find that the arginine-rich histone pair, H3 and H4, is sufficient to induce the supercoiling observed in this experiment. Both H3 and H4 are required, and in the absence of either, no other histones are effective. H3 and and H4 are as efficient, per unit weight, as a mixture of the four histones in inducing supercoils. We also show that there is a large difference between the DNA bending energy needed to form a nucleosome and that needed to form one turn of normal superhelical DNA. These two processes are energetically quite distinct and probably separable. We estimate the free energy of interaction between DNA-bound histone pairs, and find that one or two such interactions would generate enough energy to fold the DNA into a nucleosome. 相似文献