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RecA-mediated cleavage of the bacteriophage lambda repressor results in inactivation of the protein and leads to induction of the lambda prophage. Here, we report the identification of three mutations in lambda repressor that significantly increase the rate of RecA-mediated cleavage. These mutations were isolated as intragenic second-site suppressors of a mutation (ind-) which prevents cleavage. Purified repressor proteins that contain both the ind- mutation and one of the second-site mutations undergo cleavage at near wild-type rates. Purified repressors that contain the second-site mutations in otherwise wild-type backgrounds undergo RecA-mediated cleavage at significantly faster rates than wild-type, and form dimers more poorly than the wild-type protein. In related experiments, we found that other repressor mutants that dimerize poorly are also better substrates for RecA-mediated cleavage. Conversely, we show that a covalent disulfide-bonded repressor dimer is resistant to cleavage. These results support a model in which repressor monomers are the only substrate in the cleavage reaction. 相似文献
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The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
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Studies of the Ca2+ transport mechanism of human erythrocyte inside-out plasma membrane vesicles. I. Regulation of the Ca2+ pump by calmodulin 总被引:8,自引:0,他引:8
D M Waisman J M Gimble D B Goodman H Rasmussen 《The Journal of biological chemistry》1981,256(1):409-414
Calcium accumulation by human erythrocyte inside-out vesicles was linear for at least 30 min in the presence of ATP. In untreated inside-out vesicles, 3.76 +/- 1.44 nmol of calcium/min/unit of acetylcholinesterase were transported, compared with 10.57 +/- 2.05 (+/- S.D.; n = 11) in those treated with calmodulin. The amount of calmodulin necessary for 50% activation of Ca2+ accumulation was 60 +/- 22 ng/ml (+/- S.D.; n = 4). The Km (Ca2+) for calmodulin-stimulated accumulation was 0.8 +/- 0.05 microM (+/- S.D.; n = 5) using Ca2+ /ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA) buffers, or 25 microM with direct addition of unbuffered calcium. In the absence of calmodulin, these values were 0.4 and 60 microM, respectively, Km (ATP) values of 90 and 60 microM in the presence and absence of calmodulin, respectively, were measured at constant magnesium concentration (3 mM). In the presence of calmodulin, a broad pH profile is exhibited from pH 6.6 to 8.2. Maximal calcium accumulation occurs at pH 7.8. In the absence of calmodulin, the pH profile exhibits a linear upward increase from pH 7.0 to 8.2. The (Ca2+-Mg2+)-ATPase activity, measured under identical conditions, was 2.40 +/- 0.72 nmol of Pi/min/unit of acetylcholinesterase in the untreated vesicles and 11.29 +/- 2.87 nmol of Pi/min/unit of acetylcholinesterase (+/- S.D.; n = 4) in calmodulin-treated vesicles. A stoichiometry of 1.6 Ca2+/ATP hydrolyzed was determined in the absence of calmodulin; in the presence of calmodulin, this ratio was decreased to 0.94 Ca2+/ATP hydrolyzed. 相似文献