The proteins P10 and P12 have been shown to be gene products of a new stability system, ParD, of plasmid R1. It is now shown that an R1 miniplasmid, pAB112, carrying a trans-complementable amber mutation in the gene of the P10 protein, is lethal for the host in the absence of suppression. This lethal effect is suppressed in a supF background and also by deletions in pAB112 that affect the gene of the P12 protein. These data indicate that the P12 protein has a lethal effect on the host and that this effect is neutralized by the P10 protein. The possibility that the stabilization conferred by the ParD system could be due to a counterselection, mediated by P12, of cells that lose the plasmid at cell division, is discussed.

The stability determined by the systems ParD of plasmid R1 and Ccd of plasmid F is due to the concerted action of two proteins, a cytotoxin and an antagonist of this function. In this paper we report that CcdA and Kis proteins, the antagonists of the Ccd and ParD systems respectively, share significant sequence homologies at both ends. In Kis, these regions seem to correspond to two different domains. Despite the structural similarities, Kis and CcdA are not interchangeable. In addition we have shown that the cytotoxins of these systems, the Kid and CcdB proteins, do not share structural homologies. In contrast to CcdB, the Kid protein of the ParD system induces RecA-dependent cleavage of the cI repressor of bacteriophage lambda very inefficiently or not at all. The functional implications of these results are discussed.

1. The ability to oxidize quinate is elicited in Acinetobacter calco-aceticus not only by growth on quinate but also by growth on protocatechuate or its precursors such as shikimate and p-hydroxybenzoate. Accordingly the synthesis of a quinate dependent dehydrogenase was found to be induced by protocatechuate. Moreover, this enzyme seems to belong to the same coordinate block of enzymes responsible for the conversion of shikimate to /3-ketoadipyl-CoA.2. Analogies between quinate and shikimate oxidation in A . calco-aceticus have been substantiated by showing that both quinate dehydrogenase and shikimate dehydrogenase activities rely on a single enzyme.3. Several auxotrophs that have lost the ability to utilize both quinate and shikimate as sole source of carbon and energy have been isolated. Particularly relevant for this study are two of these mutants in which the dehydrogenase is severely impaired and correspondingly their ability t o oxidize the mentioned hydroaromatic compounds is substantially reduced. The properties of these auxotrophs and their revertants confirm the key role of the dehydrogenase in the dissimilation of both quinate and shikimate. The rationale of the control of this enzyme by protocatechuate is discussed.

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The linkage arrangements of genes governing glucolysis in Pseudomonas putida have been determined by transductional analysis. Five genes (gdh, kgtA, kgtB, edd and eda), comprising at least three operons, are contransducible with each other, but not with ggu (glucose and gluconate uptake) nor with genes of a known supra-operonic cluster of genes specifying enzymes of other dissimilatory pathways, nor with a biochemically uncharacterized his marker. It thus appears that P. putida may have more than one chromosomal region in which genes with dissimilatory function are clustered in a supro-operonic fashion.