On the mechanism of conjugation in Escherichia coli K 12

Summary The phenomenon of conjugation consists of many stages. The most important are: the formation of contacts between mating cells, the transfer of DNA from the donor to the recipient, and the integration of the transfered DNA fragments into the chromosome of the recipient. Only after completion of all these stages are recombinants formed. With the aid of specific inhibitiors (nalidixic acid, FUDR), thymine starvation, and use of special thermosensitive mutants it is possible to study the role of DNA synthesis during every stage of conjugation. It was demonstrated that the genetic transfer is due to semiconservative DNA-replication in the donor cell. The fragments of DNA transfered are synthesized in the period of mating by a special replication system (F-replicon). In case of T _DNA ^S mutants unable to grow at 41°, the ability to synthesize DNA during conjugation is preserved.The inhibition of the DNA synthesis in the donor cell by poisons leads to complete inhibition of genetic transfer. The third stage — formation of recombinants requires DNA synthesis in the recipient cell and is inhibited by poisoning, thymine starvation or T _DNA ^S mutations in the recipient. In cases where recombination is not involved (i.e. sexduction) the inhibition of DNA synthesis in the recipient has no significant effect.     

Identification of tomato bushy stunt virus in cherry and plum trees showing fruit pitting symptoms

The fruit pitting symptoms on cherries, plums and prunes were investigated from the standpoint of their etiology. Tomato bushy stunt virus (TBSV) was isolated from pitted fruits of these plants and from their leaves and identified by means of biological and serological methods. Both isolates reacted with antisera againstPetunia and artichoke strain of this virus. In addition, the etiology of pseudopox disease of plum and that of cherry detrimental canker is discussed.     

Purification and properties of arabis mosaic and tomato bushy stunt viruses isolated from lilac (Syringa vulgaris L.)

The paper gives more detailed characteristics of Arabis mosaic virus (AMV) and tomato bushy stunt virus (TBSV) isolated from lilac, the latter being identified in lilac (from plants suffering from yellow ring disease) for the first time. The isolate of TBSV from lilac, from which an antiserum with a titre of 1024 was prepared, is closely related to the artichoke strain. Information is given about two types of ringspot disease and about chlorotic ringspot of lilac. Whereas in the leaves of lilac suffering from ringspot disease (of ring mosaic type) the presence of AMV was demonstrated, the sap transmission from the leaves diseased with ringspot of linepattern (and wave-like mosaic) type failed; from the leaves affected by chlorotic ringspot a mixture of AMV and cherry leaf roll virus was identified. In addition, the polyetiological nature of “spring” mosaic and necrotic mosaic of lilac, in which bacteriumPseudomonas syringae van Hall, was found is dealt with. The TBSV was also identified in the isolate of necrotic mosaic.Additional index words: Lilac ringspot, chlorotic ringspot, yellow ring, “spring” mosaic, necrotic mosaic, cherry leaf roll virus,Pseudomonas syringae van Hall.     

Scale of the genetic map and genetic control of recombination after conjugation in Escherichia coli K-12.

Summary There exist many regions on the genetic map of E. coli, remarkable for very high frequency of genetic exchanges between the donor and recipient chromosome after conjugation. We call these regions fre (frequent recombination exchange). Two of them were localized: frel near to the gene tsx and fre2 adjacent to metB. The conjugational transfer of fre is characterized by high negative interference in the corresponding region of the map.The effect called Fre is genetically determined. It is slightly present on the Rec BC pathway of recombination and becomes drastic on the Rec F pathway. The effect is sharpened by an increase of temperature till 43° C during and after conjugation. The effect is absolutely dependent on the genes recA and recF.It is assumed that region fre contains many hot spots of recombination, i.e. sites of initiation, where a recF-dependent endonuclease starts the process. The scale of the genetic map of E. coli K-12 in the areas not including the fre regions is about 24 min both on the Rec BC and the Rec F pathways. In the regions including fre, the saale drops to 5 min on the Rec BC pathway and to about 1 min on the Rec F pathway. These strong variations explain the discrepancies in the mapping distances found in different works.If a plasmid F' containing the fre region is transmitted during conjugation it becomes extremely unstable. A fragment of DNA containing the fre region is always lost from the plasmid. It leads to its shortening or sometimes to the killing of the cell. The Fre effect is seen also in P1 transduction. These facts pose many questions. Suggestive answers are discussed.     

Recombinational instability of F'' plasmids in Escherichia coli K-12: localization of fre-sites

Summary The F plasmids ORF-1 (purE ⁺ tsx ^s proC ⁺ lac ⁺) and F14 (agrE ⁺ metB ⁺ ilv ⁺) contain active regions of recombination, fre I and fre II correspondingly. The plasmid ORF-1 is stable in recF ^– cells. (i.e., with the RecBC pathway of recombination) and decays in rec ⁺ cells (RecBCF pathway) giving two types of product: F⁺ and plasmid pCK-1 (tsx ^s proC ⁺ lac ⁺) containing part of the initial DNA. They are extremely instable in the presence of the RecF pathway, (recBC ^– sbcB ^–), yielding F⁺ and plasmid pCK-2 (proC ⁺ lac ⁺). The instability of plasmids depends on a region of homology between the chromosome and the episome. The instability of ORF-1 shows the participation of IS3 elements (_{1 3} and _{3 1}) in the recA, recF-dependent recombinational decay and allows localization of two active sites on the chromosome: fre I1 between purE and tsx markers and fre I2 between tsx and proC.The plasmid F14, in accordance with published data, is able to yield F⁺ cells by recA-independent recombination. But eventually this plasmid may undergo a recA, recF-dependent decay. Genetic analysis of these events allows localization of an active point of recombination, fre II1, between argE and metB. Another active point is localized inside the F factor. The recA-dependent decay of plasmid F-14 is also excluded on the RecBC pathway (recF ^– strains).     

Effect of Lincocin Treatment on the Greening Process in Bean (Phaseolus vulgaris) Leaves

The effect of lincocin (a plastid protein synthesis inhibitor) treatment on the greening process of bean (Phaseolus vulgaris L.) leaves have been studied. In comparison with control leaves treated ones had a decreased rate of chloroplast development. They had a marked chlorophyll deficiency and a decreased chlorophyll a/b ratio. Some long and short wavelength forms of chlorophyll a were lacking as evidenced from the absorption spectra at 25°C and the fluorescence spectra at 77°K. The –¹⁴CO₂ fixation was inhibited by 80–90% in treated leaves. The fluorescence induced by the measuring light was greater in the treated leaves than in the control ones, and the kinetics of the decline of the relative fluorescence intensity were also different. Electron microscopic studies showed macrogranum-like structures and incomplete membrane vesicles in the treated plastids. After longer treatment a destruction of membranes was observed. The results indicate some structural and functional membrane deficiencies and instability of the membranes.     

Etiology of two virus diseases of lilac (Syringa vulgaris L.) occurring in Czechoslovakia

Virus origin of lilac ringspot and chlorotic ringspot of lilac was identified serologically by means of double gel diffusion method. The former of these diseases is due to Arabis mosaic virus, the latter to a mixture of Arabis mosaic virus and cherry leaf roll virus. The occurrence of these viruses has been detected for the first time in Czechoslovakia.     

Genetic determination of the donor properties in Escherichia coli K-12. Phenomena of chromosome mobilization and integrative suppression.

Summary The chromosome mobilization is the ability of F⁺ donors to introduce part of the chromosome besides F-plasmids into the recipient cells during conjugation. We studied the genetic determination of this phenomenon. Most efficient almost like true Hfr's are F⁺-cells of the genotype recBC ^- sbcB ^- belonging to the RecF-recombination type. Their ability to chromosome mobilization is 50 fold higher comparing with wild type F⁺ (of the RecBC-recombination type). This property is fully dependent on the recF gene but does not depend on recL. The donors recBC ^-F⁺ but without the mutation sbcB ^- act in mobilization about 4 times weaker than wild type. Hence we see two main levels of mobilization, quantitatively very different: a recF-dependent and recBC-dependent. Both reveal an absolute requirment of the product of recA gene.The efficiency of mobilization of different markers along the chromosome was studied and mapped. The maps were identical, in spite of great difference in absolute frequencies for the RecF- and Rec BC-pathways. They are not at all random. The sites of mobilization are coinsident with the points of interaction of the F-factor leading to stable Hfr's. Therefore it is suggested that these sites of predominant mobilization are IS-sequences and that during chromosome mobilization single-strand integration of the F-factor via a semichiasmus is effected. It gives a pulse to initiate DNA transfer into the recipient but is unstable and transient and does not yield true Hfr's.The suppression of the Dna^ts phenotype in F⁺ cells due to the integration of an F-plasmid into the chromosome (integrative suppression) is increased manyfold on the RecF-pathway of recombination. Probably it is a manifestation of mentioned hot spots of recombination.The regions fre described earlier (Bresler et al., 1978) and confirmed in this paper are regarded as substrates of some recF-dependent endonuclease of recombination. Probably they coinside with clusters of IS-sequences.     

Lecithotrophic development and metamorphosis in the Indo-West Pacific brittle star Ophiomastix venosa (Echinodermata: Ophiuroidea)

Summary

The larval development of the ophiocomid ophiuroid Ophiomastix venosais described using SEM. The gastrula transforms into a uniformly ciliated early larva which progressively changes into a lecithotrophic late premetamorphic larva with a continuous bilateral ciliated band. This stage is short-lived and equivalent to a highly reduced ophiopluteus. Comparisons between O. venosa and other ophiuroid species whose development has been investigated suggest that, whatever the developmental mode (lecithotrophic or planktotrophic), a pluteus stage always occurs in ophiuroids with planktonic development. Two metamorphic stages were identified, the late metamorphic larva differing from the early one by the closure of the larval mouth. The appearance of the permanent mouth marks the end of the metamorphosis. The postlarva still possesses remnants of larval features. The transformation of the reduced ophiopluteus into a barrel-shaped metamorphic larva with transverse ciliated bands, a vitellaria larva, is followed. The possible occurrence of a unique type of metamorphic larva in non-brooding ophiuroids is discussed. Verification of this, however, needs further SEM investigations on metamorphic larva from species having “regular” planktotrophic development.