A mixed population, enriched and established in a defined medium, from a sewage sludge inoculum was capable of complete mineralization of 4-chlorobenzoate. An organism, identified as Arthrobacter sp., was isolated from the consortium and shown to be capable of utilizing 4-chlorobenzoate as the sole carbon and energy source in pure culture. This organism (strain TM-1), dehalogenated 4-chlorobenzoate as the initial step in the degradative pathway. The product, 4-hydroxybenzoate, was further metabolized via protocatechuate. The ability of strain TM-1 to degrade 4-chlorobenzoate in liquid medium at 25°C was improved by the use of continuous culture and repeated sequential subculturing. Other chlorinated benzoates and the parent compound benzoate did not support growth of strain TM-I. An active cell extract was prepared and shown to dehalogenate 4-chloro-, 4-fluoro-, and 4-bromobenzoate. Dehalogenase activity had an optimum pH of 6.8 and an optimum temperature of 20°C and was inhibited by dissolved oxygen and stimulated by manganese (Mn2+). Strain improvement resulted in an increase in the specific activity of the cell extract from 0.09 to 0.85 nmol of 4-hydroxybenzoate per min per mg of protein and a decrease in the doubling time of the organism from 50 to 1.6 h.

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S U M M A R YLipopolysaccharide (LPS) of Pseudomonas morsprunorum was extracted with hot phenol and purified by repeated centrifuging followed by either block electrophoresis or gel filtration. LPS from a virulent isolate exhbited specific phage inactivation (PIso = 0.05 ,ug LPS ml-l), whereas LPS from an avirulent phageresistant mutant did not. LPS was considered pure when a single band was detected following sodium dodecyl sulphate-cellulose acetate electrophoresis (PH 7-4). It was not phytotoxic when inoculated into cherry leaves at concentrations up to I mgml-l, but produced weak chlorosis in bean and tobacco at 2 mg ml-l : no visible symptoms appeared after treatment with lower concentrations. The chemical composition of the LPS was partly determined. I N T R O D U C T I O NThere is a strong correlation between phage sensitivity and host specificity in the English isolates of Pseudomonas morsprunorum, the cause of cherry canker (Crosse & Garrett, 1970). Isolates from cherry are sensitive to A7 and related phages (Crosse & Garrett, 1963) and infect cherry trees (Prunus avium) through the leaf scars. Almost all plum strains are insensitive to these phages and are non-virulent through cherry leaf scars. Plum strains adsorb phage A7 as readily as do cherry strains, suggesting that phage-receptor material in the bacterial wall is not involved in the host specificity of the strains (Garrett, Crosse & Sletten, I 974).Cherry-strain mutants resistant to these phages show marked attenuation of virulence. Their resistance is due to an inability to adsorb the phage, indicating a change in the receptor-site structure (Garrett et al., 1974). Hence, receptor-site material may determine the virulence of P. morsprunorum as plum and cherry isolates adsorb the phage and are both virulent on their respective hosts.Lipopolysaccharide (LPS) of Gram-negative bacterial walls has frequently been reported as phage-receptor material (e.g. Jazwinski, Lindberg & Kornberg, 1975) and also as an endotoxin (e.g. Hawiger, Hawiger & Timmon, 1975). In view of this, lipopolysaccharide was extracted from P. morsprunorum to investigate its role in pathogenicity and as receptorsite material for phage A7 and related phages. pH 71 to which one drop of antifoam (polypropylene glycol 2000; Shell Research, Sittingbourne, Kent) was added before autoclaving. The cultures were stirred and aerated at 18 "C. Air was supplied at 500 cm3 min-l via a sintered-glass gas distribution tube (porosity 3). The purity of the cultures was tested by the Gram-stain method and by subculturing on nutrient sucrose agar (NSA; Crosse, 1959). The optimum temperature for growth of c28 is normally 25 "C but when grown in broth with efficient aeration and stirring at this temperature, cells grew to 4 to 5 times their normal length. These cells quickly reverted to the normal size when aeration was reduced and were not elongated when grown at 18 "C. Cells were harvested at the late-exponential phase using a continuous-flow centrifuge at 4 "C, and washed twice with cold distilled ...