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S Dennison

Publications and source records attributed to S Dennison.

11 recordsLinked to original sources
PubMed1975

Conjugational behaviour of N plasmids in Escherichia coli K12.

R factors of the N incompatibility group apparently transfer at low frequencies (often 10(-6)-10(-5) per donor cell) in 30 min liquid matings. However, the level of transfer is greatly increased when donor-recipient mixtures are held on solid medium selective for the recipient only, prior to transconjugant selection. Increase in transconjugant recovery is still donor-dependent at plating and therefore arises from mating on the plate. The process limiting N-mediated conjugation in liquid is probably mating pair formation, suggesting that the increase in yield of transconjugants may result from provision of a solid substrate for mating pair formation rather than from the delay in selection per se. The kinetics of N plasmid transfer on solid medium resemble those of traniments suggest that RP4, the prototype P incompatibility group plasmid, shows a stimulation of transfer on solid medium similar to the N plasmids studied. We suggest that N, and probably P, plasmids are (like F and the col V's) naturally derepressed for fertility, but this is masked by the imcompetence in liquid matings of donors carrying them.

Coliphages↗
PubMed1975

Variation in expression of sex factor genes in the Proteus-Providencia group relative to Escherichia coli.

Several instances of anomalous expression of genes introduced from Escherichia coli K-12 into Proteus mirabilis have been described. It is shown here that control of sex pilus synthesis directed by the F-like R factor R1 and its depressed derivatives R1-16 (O-C) and R1-19 (i-minus) is also anomalous in P. mirabilis. Piliation in cells bearing the depressed plasmids is expressed at a lower level than in E. coli K-12, and repression is absent in R1-carrying cells. Preliminary results show a similar effect in Providencia. In Proteus morganii, a similarly reduced level of piliation in R1-16-+ or R1-19-+ cultures is observed, but an intermediate level of repression occurs in R1-+ cultures. Less extensive data suggest that expression of the sex factor genes of an R factor of the N incompatibility group differs far less between E. coli and P. mirabilis hosts. Possible bases for these effects are discussed.

Coliphages↗
PubMed1972

Host specificities of RNA phages.

Host ranges of members of four groups of male-specific RNA coliphages were determined by plating on hosts carrying various derepressed plasmids. An RNA phage originally isolated on Pseudomonas aeruginosa failed to form plaques on any of the strains of Escherichia coli.

Coliphages↗
PubMed1972

Naturally occurring R factor, derepressed for pilus synthesis, belonging to the same compatibility group as the sex factor F of Escherichia coli K-12.

A naturally occurring R factor with constitutive pilus synthesis is described which resembles the sex factor F in compatibility and in restricting coliphage T7. Unlike F, it is not cured during growth with acridine orange. Results suggest that the R factor produces repressor of pilus synthesis, to which the operator is insensitive (i(+)o(c)). In this respect it differs from both the F factor (i(-)o(+)) and wild-type F-like R factors (i(+)o(+)).

Acridines↗