Replication of adenovirus mini-chromosomes

We have isolated adenovirus origins of DNA replication from both the right and left ends of the genome, which are functional on linear autonomously replicating mini-chromosomes. The mini-chromosomes contain two cloned inverted adenovirus termini and require non-defective adenovirus as a helper. Replicated molecules are covalently attached to protein, and DNA synthesis is initiated at the correct nucleotide even when the origins are not located at molecular ends. The activity of embedded origins leads to the generation of linear minichromosomes from circular or linear molecules. These observations therefore suggest that sequences within the adenovirus origin of replication position the protein priming event at the adenovirus terminus. Experiments investigating the regeneration of deleted viral inverted terminal repeat sequences show a sequence-independent requirement for inverted sequences in this process. This result strongly suggests that repair results from the formation of a panhandle structure by a displaced single strand. On the basis of these observations we propose a model for the generation of adenovirus mini-chromosomes from larger molecules.     

Complete nucleotide sequences of all three poliovirus serotype genomes. Implication for genetic relationship, gene function and antigenic determinants

The complete nucleotide sequences of the genomes of the type 2 ( P712 , Ch, 2ab ) and type 3 (Leon 12a1b ) poliovirus vaccine strains were determined. Comparison of the sequences with the previously established genome sequence of type 1 (LS-c, 2ab ) poliovirus vaccine strain revealed that 71% of the nucleotides in the genome RNAs were common, that the 5' and 3' termini of the genomes were highly homologous, and that more than 80% of the nucleotide differences in the coding region occurred in the third letter position of in-phase codons, resulting in a low frequency of amino acid difference. These results strongly suggested that the serotypes of poliovirus derived from a common prototype. A comparison of the amino acid sequences predicted from the genome sequences showed highest variation in the capsid protein region, whereas non-structural proteins are highly conserved. Initiation of polyprotein synthesis occurs in all three strains more than 740 nucleotides downstream from the 5' end. An analysis of the non-coding region suggests that small peptides that could potentially originate from this region are conserved. The amino acid sequences immediately surrounding the cleavage signals, however, show a higher than average degree of variation. The analysis of the amino acid sequences of the capsid protein VP1 of all serotypes has led to the prediction of potential antigenic sites on the virion involved in neutralization.     

Temperature as a determinative factor in the evolution of genetic systems

Summary Heat induces a number of premutational lesions (for example, the deamination of cytosine to uracil) in DNA and RNA. These kinds of errors occur in resting as well as replicating polynucleotides. However, an increase in temperature also raises the probability of copying error occurring in nucleic acids because of increased thermal noise in the replicative machinery. In most modern genetic systems, the majority of heat-induced lesions are efficiently repaired. It follows that the importance of heat-induced error increases as the effectiveness of repair declines. We show in this paper that the error rate of enzymatic polynucleotide copying is expected to increase monotonically with temperature. We also explore the effects of temperature variations on the early evolution of biological information transmission mechanisms.     

水分胁迫下露花光合型的转变

CAM植物按其对环境的反应可分为两种类型:即专一CAM植物和兼性CAM植物。前者不易受外界环境的变化而改变其CAM性质;后者的光合型可随季节和水分胁迫等而发生变化,也可因人工诱导作用而使其由C_3型转变为CAM型。     

Analysis of fluorescence transients of DCMU-treated leaves of Triticum species to provide estimates of the densities of photosystem II reaction centres

The fluorescence of the chlorophyll associated with photosystem II was studied in seedling and flag leaves of Triticum species. Seedling leaves of the diploid species T. urartu had higher values of t (the normalised area over the fluorescence induction curve of DCMU treated leaves) than those of the other species studied which included hexaploid T. aestivum. However this difference was not evident when leaves were grown in a low light intensity (40 µmol quanta of photosynthetically active radiation m^–2 s^–1). The smaller total number of chlorophyll molecules per photosystem II reaction centre (chl/RCII) in T. urartu (177) as compared with the other species (mean 234) was deduced from the observed differences in t. As a consequence of its lower chl/RCII, despite slightly lower chlorophyll content (mg m^–2), T. urartu had a greater density of reaction centres than the other species (2880 cf 2230 nmol m^–2 of leaf). Consistent with the lower chl/RCII of T. urartu, it had a higher chlorophyll a/b ratio than the other genotypes. Seedling leaves of T. urartu had higher light saturated rates of photosynthesis than those of the other species, when grown at high light, a difference associated with reaction centre density.In flag leaves, when the complications due to variable development and senescence patterns were eliminated, t of the diploid species including T. urartu was lower than that of T. aestivum. The lower apparent chl/RCII of T. urartu arose partly because the molar extinction coefficient of the chlorophyll in the leaves of T. urartu was greater than in T. aestivum. However, the density of PS II reaction centres was slightly lower for the diploid species studied because their chlorophyll contents were lower than the hexaploids.The validity of the method for estimating chl/RCII from fluorescence transients is discussed. The possibility is considered that the difference in apparent chl/RCII of flag and seedling leaves of R. urartu as compared to the other five genotypes is a consequence of its different adaptive response to the spectral quality of the light.     

Chlorophyll-protein and polypeptide composition of Mn-deficient sugar beet thylakoids

The chlorophyll-protein and polypeptide composition of manganese deficient and control sugar beet thylakoids was examined using three different detergent-electrophoresis systems. On a per chlorophyll basis, manganese deficiency reduced the amounts of CPa complex (separated by sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis), and CP 47 and CP 43 complexes (separated by octylglucoside/SDS-polyacrylamide gel electrophoresis) without decreasing the amounts of light harvesting complexes. Lithium dodecylsulfate/Triton X-100 polyacrylamide gel electrophoresis showed that manganese deficiency decreased several thylakoid polypeptides, including a chlorophyll b containing 30 kilodalton chlorophyll-protein complex, but did not decrease the amounts of 28 and 29 kilodalton light-harvesting chlorophyll b-containing polypeptides.     

Thermodynamics of the charge recombination in photosystem II

The temperature dependence of the electric field-induced chlorophyll luminescence in photosystem II was studied in Tris-washed, osmotically swollen spinach chloroplasts (blebs). The system II reaction centers were brought in the state Z⁺P⁺-Q_A ^-Q_B ^- by preillumination and the charge recombination to the state Z⁺PQ_AQ_B ^- was measured at various temperatures and electrical field strengths. It was found that the activation enthalpy of this back reaction was 0.16 eV in the absence of an electrical field and diminished with increasing field strength. It is argued that this energy is the enthalpy difference between the states IQ_A ^- and I^-Q_A and accounts for about half of the free energy difference between these states. The redox state of Q_B does not influence this free energy difference within 150 s after the photoreduction of Q_A. The consequences for the interpretation of thermodynamic properties of Q_A are discussed.Abbreviations DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - I intermediary electron acceptor - Mops 3-(N-morpholino)propanesulphonic acid - P (P₆₈₀) primary electron donor - PS II photosystem II - Q_A and Q_B first and second quinone electron acceptors - Tricine N-tris(hydroxymethyl)methylglycine - Tris tris-(hydroxymethyl)aminomethane - Z secondary electron donor Dedicated to Professor L.N.M. Duysens on the occasion of his retirement     

Detection of rapid induction kinetics with a new type of high-frequency modulated chlorophyll fluorometer

A newly developed modulation fluorometer is described which operates with 1 sec light pulses from a light-emitting diode (LED) at 100 KHz. Special amplification circuits assure a highly selective recording of pulse fluorescence signals against a vast background of non-modulated light. The system tolerates ratios of up to 1:10⁷ between measuring light and actinic light. Thus it is possible to measure the dark fluorescence yield and record the kinetics of light-induced changes. A high time resolution allows the recording of the rapid relaxation kinetic following a saturating single turnover flash. Examples of system performance are given. It is shown that following a flash the reoxidation kinetics of photosystem II acceptors are slowed down not only by the inhibitor DCMU, but by a number of other treatments as well. From a light intensity dependency of the induction kinetics the existence of two saturated intermediate levels (I₁ and I₂) is apparent, which indicates the removal of three distinct types of fluorescence quenching in the overall fluorescence rise from F₀ to F_max.Abbreviations Q_A and Q_B consecutive electron acceptors of photosystem II - PS II photosystem II - P 680 reaction center chlorophyll of photosystem II - F₀ minimum fluorescence yield following dark adaptation - F_max maximum fluorescence yield - DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethyl-urea - DCCD N,N-dicyclohexylcarbodiimide - PQ plastoquinone - DAD diaminodurene Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.     

Interaction of nucleic acids with the DNA-dependent RNA polymerases of Drosophila

The interaction between the three Drosophila DNA-dependent RNA polymerases (EC 2.7.7.6) and the DNA template or the RNA product was investigated by photochemical cross-linking and binding studies, using RNA polymerase subunits immobilized on nitro-cellulose filters. It can be shown that the two largest subunits are responsible for the binding of the enzymes to both template and newly-synthesized RNA.