Isolation and characterization of Hfr males in Citrobacter freundii

Donor strains of the soft-rot bacterium Erwinia chrysanthemi and conjugational transfer of the pectolytic capacity

Donor strains of Erwinia chrysanthemi ICPB EC16, a member of the soft-rot (pectolytic) section of the enterobacterial genus Erwinia, were obtained by chromosomal integration of an F'lac(+) plasmid originating from Escherichia coli. These stable donor strains, selected from an unstable F'lac(+) heterogenote by repeated platings of single Lac(+) colonies on lactose minimal agar, do not segregate (as does the parent F'lac(+) heterogenote) into Lac(-) or F(-) clones, in either the presence or absence of acridine orange. One representative donor strain (from the 12 that have been selected) has been examined in more detail; it can transfer ade(+), gal(+), gtu(+) (utilization of galacturonate), his(+), lac(+), leu(+), lys(+), mcu(+) (multiple carbohydrate utilization), pat(+) (production of polygalacturonic acid trans-eliminase), thr(+), and trp(+) in a polarized manner to appropriate recipient strains of E. chrysanthemi; the frequencies of ade(+), leu(+), and thr(+) transfer were higher than those of the other markers tested to date. This donor strain transfers lac(+) genes during a 6-h mating on membranes; most of the Lac(+) recombinants are donors of chromosomal markers. The kinetics of entry as well as the frequencies of transfer of chromosomal markers indicate that thr(+) and leu(+) enter the recipient as proximal markers and that lac(+) enters as a distal marker. Analysis of the recombinants demonstrates close linkage between thr and leu, ade and thr, his and pat, and his and trp loci. The results suggest that the integration of F'lac(+) into the chromosome of E. chrysanthemi has occurred at a region adjacent to the leu-thr loci, and that the chromosome is transferred in the following sequence: origin----leu--thr--ade--lys--mcu--pat--his--trp--gal--gtu--lac--F. Plant-tissue maceration occurs in Pat(+) recombinants and not in Pat(-) recombinants, even though both form another pectolytic enzyme, hydrolytic polygalacturonase. This genetic evidence supports the idea that the E. chrysanthemi polygalacturonic acid trans-eliminase plays an essential role in bringing about plant-tissue maceration.

Thermolabile repression of cephalosporinase synthesis in Citrobacter freundii

An unusual regulatory system of cephalosporinase synthesis in Citrobacter freundii has been found. When the bacteria are grown at 20 C, the cephalosporinase is synthesized as a typical inducible enzyme and benzylpenicillin acts as an effective inducer. The enzyme, however, is synthesized in the absence of the inducer at growth temperatures above 25 C. when the growth temperature is shifted from 20 C to 37 C, the induction of enzyme synthesis is observed after about one half of the organism doubling time, but it does not occur in the presence of chloramphenicol. The reverse control mutants, the enzyme constitutive synthesis of which is markedly depressed by benzylpenicillin, were isolated from the C. freundii wild strain. The possibility that the enzyme synthesis is governed by a regulatory system analogous to the its mutant of the lac operon in Escherichia coli was suggested.

Genetics of colicin E susceptibility in citrobacter freundii

The insensitivity of Citrobacter freundii to the E colicins is based on tolerance to colicin E1 and resistance to colicins E2 and E3. Spontaneous colicin A resistant mutants of C. freundii also lost their colicin E1 receptor function. Sensitivity to colicin E1 can be induced by F'gal+tol+ plasmids, the tol A+ gene product of which is responsible for this effect. Receptor function for colicins E2 and E3 is induced by the E. coli F'14 bfe+ plasmid, which is also able to enhance notably the receptor capacity for colicin E1. The bfe+ gene product of E. coli, which is responsible for these phenomena, also restores the receptor function for colicin A and E1 in colicin A resistant mutants of C. freundii. All results show that there is a remarkable difference between the E. coli bfe+ gene product and the bfe+ gene product of C. freundii and also between the tol A+ gene products of these strains. The sensitivity to phage BF23 parallels the sensitivity to colicins E2 and E3 and is also induced by the F'14 bfe+ plasmid.

A genetic study of tolerance and resistance to colicin A in Citrobacter freundii

Colicin A-insensitive mutants of Citrobacter freundii were isolated and grouped into six phenotypic classes characterized by sensitivity, insensitivity or partial insensitivity to the bacteriocins S6, DF 13 and colicin A, and sensitivity or insensitivity to deoxycholate (DOC) and ampicillin. Mapping by the gradient-of-transmission method revealed the chromosomal regions in which the responsible genes are situated. Res-3 mapped near pur between pur and thr; Tol-5 mapped between aro and ilv and Tol-4 between gal and pur; Tol-1, Tol-2 and Tol-3 are situated close to gal. All the mutations that mapped near gal rendered the bacteria more sensitive to DOC and ampicillin. Complementation analysis with E. coli plasmids showed that the three phenotypic groups that map near gal were complemented by E. coli plasmids and fall into three complementation groups. Two of these are quivalent with the tol A and tol B genes in E. coli.