A comparison has been made between the physiology and amino acid sequence of the lactose carriers of Klebsiella pneumoniae M5a1 and Escherichia coli K-12. The membrane transport of lactose was much weaker in Klebsiella than in E. coli. On the other hand o-nitrophenylgalactoside uptake by Klebsiella was distinctly greater than with E. coli. In spite of the differences in sugar transport between the two organisms, the amino acid sequences of the respective lactose carriers were remarkably similar (60% of the amino acids are identical).

SUMMARYCurrently favoured models postulate that gene conversion is due to the correction of mis-matches in heteroduplex DNA. If heteroduplex is formed reciprocally on both chromatids participating in recombination, the mis-matches due to a heterozygous site will be different on the two chromatids, and there will be four correction probabilities to be taken into account. It is shown that, given the frequencies of the five different kinds of aberrant ascus ratios, it is possible to calculate four alternative sets of values for the four correction probabilities and the total number of asci in which heteroduplex is formed. These four solutions reduce in effect to two when there are no other markers distinguishing the two chromatids. With the aid of flanking markers and the assumption that heteroduplex formation is chemically polarized, it is possible, in principle, to choose one best solution.The method has been applied to the five one-point crosses inSordaria fimicolafrom which most data are available. The data from four different mutants crossed to wild type are compatible with a restricted model in which the correction frequencies, from mutant to wild and from wild to mutant, are the same on both chromatids. In the case of the fifth mutant the data are not consistent with this restricted model, and indicate different correction frequencies in the two chromatids.

SUMMARYKlebsiella aerogenesV9A carrying alacplasmid in addition to its chromosomal operon showed strongly positive fermentation of lactose on MacConkey lactose agar plates, and was found to transport the lactose analogue thiomethyl-β-galactoside (TMG) at a rapid rate. The strain that had been freed of the plasmid showed moderate transport due to the chromosomallacoperon. When a plasmid bearing a mutation inlac Ywas inserted into a strain with a normalYgene, the resulting diploid became lactose-negative in phenotype. The presence ofE. coliF′lacfactors that carriedlac Ymutations, whether deletions or missense or nonsense mutations, also renderedlac Y+Klebsiellalactose-negative. Such diploids, after growth in 1% lactose, transported TMG at a much lower rate than the corresponding plasmid-freelac Y+Klebsiella. However, this interference bylac Y−plasmids with the expression of the chromosomallac Ygene was not seen when cells were induced with IPTG or when the chromosomal and plasmidlacoperons were both constitutive. It was found that this effect of the plasmids was dependent on their possessing an intactlacZgene.

Klebsiella aerogenes V9A carrying a lac plasmid in addition to its chromosomal operon showed strongly positive fermentation of lactose on MacConkey lactose agar plates, and was found to transport the lactose analogue thiomethyl-β-galactoside (TMG) at a rapid rate. The strain that had been freed of the plasmid showed moderate transport due to the chromosomal lac operon. When a plasmid bearing a mutation in lac Y was inserted into a strain with a normal Y gene, the resulting diploid became lactose-negative in phenotype. The presence of E. coli F′ lac factors that carried lac Y mutations, whether deletions or missense or nonsense mutations, also rendered lac Y + Klebsiella lactose-negative. Such diploids, after growth in 1% lactose, transported TMG at a much lower rate than the corresponding plasmid-free lac Y + Klebsiella . However, this interference by lac Y − plasmids with the expression of the chromosomal lac Y gene was not seen when cells were induced with IPTG or when the chromosomal and plasmid lac operons were both constitutive. It was found that this effect of the plasmids was dependent on their possessing an intact lacZ gene.

The Tc determinants responsible for R plasmid-mediated resistance to tetracycline carry a resistance gene and a repressor gene, here designated tetA and tetI. With derepressed (tetI−) mutants of the Tc determinant in the plasmid R57, it is shown that R6-5, which has lost expression of Tc resistance through insertion of IS3 into or near tetA, retains an active tetI gene and is therefore a source of tet repressor product without the resistance protein. Tcr revertants of R6-5, presumed to have regained resistance by excision of IS3, are also found to retain an active tetI gene.

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SUMMARY A series of derepressed mutants of the tetracycline resistance (T) determinant in R-factor R57 have been found to be repressor-negative and recessive to the T determinant in R6. It is shown that these (Tdr) mutants are dominant to the inducible T determinant in RPl, indicating that the T determinants in R57 and RP1 code for different repressors of the resistance gene. The same Tdr determinants are unstable in cells carrying both the R57 mutant and RP1, probably due to selection against the dominant Tdr gene because it depresses the growth rate of the host cell compared with its T + homologue. It is suggested that the T determinants giving high-level resistance in R57, R6 and R100 form one homologous group, probably disseminated by the transposon TnlO, while T determinants giving a much lower level of resistance, such as that in RP1, form a separate group, which may include those in R46, and R199. It is proposed that the gene responsible for tetracycline resistance should be designated tetA and the repressor gene tetI . The R57 Tdr mutants then have the genotype tetI − tetA + .

Klebsiella strain RE1544 contains two lac operons, one on the chromosome and one on a lac plasmid. A mutant of RE1544, in which the lacZ genes of both operons produce no active enzyme, was found to synthesize a beta-galactosidase that hydrolyzes ortho-nitrophenyl-beta-D-galactopyranoside but not lactose. Synthesis of this beta-galactosidase (BGase-III) is induced by lactose but not by isopropyl-1-thio-beta-D-galactopyranoside or methyl-beta-D-thiogalactopyranoside. In both the regulation of synthesis and substrate specificity, BGase-III strongly resembles the ebg0 enzyme of Escherichia coli. Nevertheless, by the criteria of immunological cross-reactivity and subunit molecular weight, BGase-III is not related to the ebg0 enzyme.

SUMMARYThe Lac plasmids in twoKlebsiellastrains, V9A and RE1544, give characteristic Lac-permease defective mutations which produce a lactose-negative (ML−) phenotype in the host strain, in spite of the presence of alacoperon on the chromosome. These ML−clones can revert to the wild-type (ML+) phenotype, and the mechanism of this reversion is examined. In V9A carrying its own Lac plasmid (FKlac), it is shown that reversion of ML−to ML+is usually the result of mutation to constitutivity of one of two galactoside permeases which are not induced by lactose but accumulate lactose when otherwise induced or made constitutive. However, in one out of the 51 ML+revertants tested the mechanism of reversion appeared to be a change back to wild type of the Lac plasmid's own permease gene. In V9A carrying the Lac plasmid (pRE6) from RE1544, successive changes of phenotype were obtained of ML+to ML−to ML+to ML−to ML+; these were all found to be the result of changes in the plasmid permease gene, and could be simply explained if an IS sequence could insert and excise from this gene.

SUMMARYEleven mutants of R-factor R57 have been isolated which show constitutive expression of resistance to tetracycline (Tc). These derepressed (Tdr) mutants all gave a much greater resistance to Tc and to its analogue, minocycline, than could be obtained by optimal induction of cells carrying the wild-type (T+) determinant. Cells carrying each of the Tdr mutants together with T+of either R6-S or of a plasmid found inEscherichia colimi19 showed inducible Tc resistance, indicating that the Tdr mutants were all recessive, i.e. of repressor-negative type. Tdr1 was not recessive to the T-determinant of RP1, suggesting that the repressor gene products of the T-determinants in R57 and RP1 have different specificities.

SUMMARYKlebsiella aerogenesV9A contains twolacoperons, one chromosomal and one borne by the Lac plasmid,FKlac. It is shown that the plasmid Lac repressor resembles that ofEsherichia coliin that it represses theE. colioperon but is inactivated by melibiose. In contrast, the chromosomal Lac repressor of V9A is unable to repress theE. colioperon or the FKlacoperon, but is converted into an active repressor of both these operons on becoming bound to melibiose. The chromosomal Lac repressor is able to repress its own operon whether or not it is bound to melibiose, but is inactivated by isopropyl-β-D-thiogalactoside. These results support the hypothesis that theKlebsiellaLac plasmid was derived from an ancestralE. colichromosome.

SUMMARYA variety ofKlebsiellastrains examined all show either a strong (ML+) or a weak (ML−/+) lactose-positive phenotype on MacConkey agar. ML+Klebsiellae have about 10 times the β-galactosidase activity of ML−/+strains in cultures both induced and not induced for this enzyme. Of 14 ML+strains of diverse origin tested, at least 13 carry alacoperon on a plasmid which can be transferred toEscherichia coli. The seven plasmids so far studied in detail all belong to the F compatibility group but are unable to promote their own transfer. To explain these results it is suggested that theKlebsiellagroup derive from a common ancestor with a chromosomallacoperon of low efficiency, which was made good by the acquisition of alacoperon from another bacterial strain, probablyE. coli: the newlacgenes remained as a plasmid, possibly because they could not be integrated in the new host.

SUMMARYInKlebsiella aerogenesstrain V9A melibiose and raffinose fail to induce thelacoperon and strongly repress its induction by isopropyl-β-d-thiogalactopyranoside (IPTG). This repression is specific to thelacsystem and is not released by cAMP, which releases the catabolite repression exerted by glucose and sucrose. It is concluded that melibiose and raffinose bind to thelacrepressor molecule inKlebsiellaV9A, competitively displacing IPTG, and that the resulting complex is able to repress thelacoperon. OtherKlebsiellastrains tested show the same repression phenomenon.