摘要:

SUMMARY1This article is concerned with the molecular basis of host‐obligate‐parasite interactions that lead to either resistance or susceptibility of the host plants to obligate fungal pathogens.A scheme of molecular events that may occur during the primary stages of host‐pathogen interactions is proposed. It implies derepression of novel genes, intercistronic complementation of oligomeric enzymes in the host and the pathogen and, in some cases, complementation between host‐specific and pathogen‐specific polypeptides leading to the formation of catalytically different enzymes whose functions are essential for the continuation of the host‐parasite relationship.2The basic tenets of the scheme are (a) transfer of nucleic acids, (b) synthesis of new RNA and proteins, (c) subunit interactions and formation of hybrid enzymes and (d) enzyme complementation during host‐parasite interactions. These are discussed in the light of some recent discoveries in plant pathology, biochemistry, virology and molecular biology.(a) Transfer of macromolecules from one organism to another at the levels of (i) DNA, (ii) RNA or (iii) polypeptides frequently occurs in nature. Transfer of polypeptides (or amplified DNA segments or mRNA molecules coding for them) from pathogen to the host cells could lead to complementation between host‐specific and pathogen‐specific subunits of oligomeric enzymes.(b) There are readily detectable changes in the transcription pattern at early stages of rust infection. Further analyses have shown that the observed changes in the pattern of gene expression are due to biochemically detectable alterations in the transcriptive specificity of RNA polymerases as well as in the template activity of chromatin. These findings are consistent with the derepression of novel genes in the host elicited by host‐pathogen interactions.Host‐pathogen interactions lead to the appearance of RNases with unique physical and catalytic properties. The appearance of these RNases coincides with (i) a remarkable increase in the rate of synthesis of all major classes of RNA molecules and (ii) an accumulation of RNA in the inoculated host suggesting an essential role of the new RNases in the post‐transcriptional processing of precursor RNA molecules.According to the postulated scheme, changes in the pattern of transcription and post‐transcriptional processing are brought about by intragenic and intergenic complementation of the enzymes involved in these processes including RNA poly‐merase and RNase molecules.(c) The phenomenon of intragenic complementation has been observed in a large number of organisms including plants and fungi. In addition, fully functional hybrids of oligomeric enzymes have been produced in vitro with subunits from different tissues, species, genera, families and even phyla.In procaryotic systems, interactions of host‐specific and phage‐specific subunits leading to the formation of new enzymes are of common occurrence.(d) In all oligomeric enzymes, relatively subtle changes in the subunit structure lead to radical changes in their catalytic and regulatory properties. This phenomenon is illustrated by the two enzyme systems, RNA polymerases and RNases.The mode of operation of the complementation model and its relevance to the host‐specificity of the pathogens and resistance or suscep

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01348.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1The succession of hypotheses on the role of myotomal muscle in the generation of swimming movements is described and the conventional concept of ‘waves of contraction’ is shown to be based on a number of misinterpretations.2The form of undulatory movements in vertebrate swimmers is characterized by the properties of the sinusoidal oscillation of parts of the body about the axis of progression. An important variable is the relative amplitude of the lateral oscillation of the head end, which can be large in some animals though usually small in most adult aquatic vertebrates.3Cinematographic records of swimming animals are examined to determine the forces involved in the generation of waves of bending. A simplified analysis suggests that undulation can be produced by alternation of tension development from side to side without ‘waves of contraction’ passing down the body.4Model systems which are able to flex from side to side are considered and two types distinguished ‐ the ‘resistance‐dominated’ which propagates waves of bending from centre to extremities, and the ‘stiffness‐dominated’ which does not. The type to which a model belongs is determined by the interrelationship of its stiffness and resistance, and the power with which it flexes.5A model homogeneous in its properties along its length cannot generate longitudinal movement by flexing from side to side. Some degree of unevenness from one end to the other is required for propulsion.6Observations of the movements of an ‘ostraciiform model’ are shown to discount previous theories of the hydromechanics of swimming by the oscillation of a stiff tail about a single pivot. A new interpretation is provided.7The majority of vertebrate swimmers behave like ‘hybrid oscillators’ which flex from side to side, ‘resistance‐dominated’ posteriorly and ‘stiffness‐dominated’ anteriorly.8The origin of the ‘waves of contraction’ suggested by electromyograms of swimming animals is traced to the requirement for a tail of variable stiffness for variable frequency of oscillation and to the need to reduce lateral oscillation of the head. Delayed contraction posteriorly and early contraction anteriorly contribute to these functions.9The ability of amphioxus to swim backwards and the inability of most vertebrates to do so is related to their structural organization in the form of ‘hybrid oscillators’.10Electromyograms are examined in the light of these mechanical models. A developmental sequence is described for the newt which illustrates the organization of the muscular control of swimming movements and may throw l

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01349.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01350.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1Many species of Gram‐negative bacteria carry a layer of regularly arranged sub‐units on the outer surface of their outer membrane. The subunits are arranged tetra‐gonally or hexagonally and have centre‐to‐centre spacings ranging from 4 to 35 nm, depending on the bacterial species.2The regularly arranged layer has been detected by electron microscopy in whole cells, cell walls, outer membranes and assemblies of isolated subunits. The regular arrays can be seen in intact cells by shadowing or freeze‐etching and, occasionally, in thin sections or after negative‐staining. Freeze‐etching and negative‐staining have been used to show the regular arrays in isolated cell walls and outer membranes. Negative‐staining is used in the examination of assembled isolated subunits.3Optical diffraction of electron micrographs provides more detailed information of the fine structure of the subunits in the regular array.4The regularly arranged surface layer can be removed by protein perturbants, by chelation of divalent cations with EDTA and EGTA, by cation substitution or by acidification.5The two surface subunits which have so far been purified have been found to be acidic proteins with molecular weights of 67000 (Acimtobacter199A) and 140000 (Spirillum serpens).6InAcinetobacter199A the surface protein is attached to the protein of the outer membrane through a salt bridge involving Ca2+or Mg2+. Evidence exists that there may be a similar mode of attachment in other species.7Isolated surface proteins fromAcinetobacter199A and fromSpirillumspp. have the ability to reassemble into the same pattern as that seen on the bacterial surface, either in isolation or in the presence of cell‐wall fragments to act as nucleating agents. Self‐assembly ofAcinetobacter199A a‐protein requires chloride ions.8Acinetobacter199A a‐protein can only be incorporated onto the bacterial surface if an intact lipopolysaccharide membrane is formed first to receive the intrinsic membrane proteins to which the a‐protein attaches. Impairment of lipopolysaccharide synthesis by bacitracin prevents incorporation of a‐protein and other membrane proteins into the outer membrane.9Continuing incorporation of pre‐formed radioactivea‐protein into the outer membrane inAcimtobacter199A can be detected for 10 min after transference from radioactive to non‐radioactive growth medium.10Spirillum metamorphum, Flexibactersp. andAcinetobacter199A synthesize more surface protein than is required to cover the cell surface. The excess is secreted into the growth medium.11The regularly arranged surface proteinof Acinetobacter199A provides partial protection against isolated lysosomal proteinases from polymorphonuclear leucocytes.Spirillumspp. is protected fromBdellovibrioinvasion by the surface protein.12Secreteda‐protein fromAcimtobacter199A has phospholipase A, activity. No phospholipase activity can be detected when the a‐

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01351.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1This paper includes observations made and references to relevant literature published since the appearance ofThe Wren(Armstrong, 1955). It includes comparisons between the Wren's behaviour and that of other birds.2The Wren is exceptional and perhaps unique in having extended its range in comparatively recent times from North America to Asia, Europe and North Africa. Fossil remains accepted as evidence that it reached Britain some 100000 years ago were wrongly identified.3The Wren is also exceptional in its adaptability. Behaviour, especially the character of the pair‐bond, is related to habitat and foraging opportunities, with polygamy occurring in favourable habitats and monogamy in more marginal areas. This facultative pair‐bond is believed to be related primarily to the food supply. The life‐style is a complex in which the size of the bird and its territory, nature of the display, song and calls, character of the food supply, structure of the nest, roosting behaviour and other adaptations constitute a delicately integrated complex, in which foraging is probably of primary importance. This principle is applicable to many species.4Size of population is very variable as numbers are greatly reduced during severe winters but the remnants surviving in prime or ‘refuge’ areas form nuclei enabling the population to expand widely during a series of mild winters, invading marginal areas where the size of territories tends to be smaller when the population reaches a peak.5Song serves various functions and is partly innate and partly learned. It is readily recognizable throughout the bird's range but local groups utter distinctive songs and there is a tendency for northerly birds to utter longer songs. Individual birds can recognize the songs of neighbours.6The number of nests built is determined by factors as different as the maturity of the male and the amount of rainfall. The enclosed nest facilitates variability in the pair‐bond as a single bird can rear at least some of the young. The complex of behaviour involved, characteristic of a number of tropical species, is further evidence of the Wren's tropical or subtropical antecedents.7There is variability as to when the eggs are laid, before or after the female's first sortie of the day, and also in the inception and duration of incubation. Published estimates of the incubation period not based on data obtained with automatic recording apparatus can be misleading.8When his mate disappears, a male who has done little to tend the nestlings may change abruptly to feeding them with extreme diligence, even spending the night in the nest with them. The male visits his nests whether or not they are occupied and is usually aware of the state of affairs at any nest.9Occasionally a Wren may visit a neighbour's nest and possibly feed the nestlings or a bird of another species may feed young Wrens but there is no evidence that, as happens with some other species, young birds feed nestlings of a later brood. Such ‘altruistic’ behaviour in some species is believed to be advantageous.10As in many other passerine species there are morning and evening peaks in foraging activity for the nestlings. The number of feeding visits tends to be related to the number of the young as well as to the vigour of their begging.11Social roosting is a form of behaviour characteristic of some tropical species of wren but which in them has no close relationship to the air temperature. Its persistence in the otherwise asocial Wren enables it to survive the low temperatures of n

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01352.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01353.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY