In members of the family Rhizobiaceae, many phenolic compounds are degraded by the protocatechuate branch of the ,I-ketoadipate pathway. In this paper we describe a novel pattern of induction of protocatechuate (pca) genes in Rhizobium leguminosarum biovar trifolii. Isolation of pca mutant strains revealed that 4-hydroxybenzoate, quinate, and 4-coumarate are degraded via the protocatechuate pathway. At least three inducers govern catabolism of 4-hydroxybenzoate to succinyl coenzyme A and acetyl coenzyme A. The enzyme that catalyzes the initial step is induced by its substrate, whereas the catabolite P-carboxy-cis,cis-muconate induces enzymes for the upper protocatechuate pathway, and j8-ketoadipate elicits expression of the enzyme for a subsequent step, 13-ketoadipate succinyl-coenzyme A transferase. Elucidation of the induction pattern relied in part on complementation of mutant Rhizobium strains by known subclones of Acinetobacter genes expressed off the lac promoter in a broad-host-range vector.

Cent re fo r Rhizobium Studies, Division Of Science, M u rdoc h U n ive rsity, Murdoch, Perth, Western Australia 61 50 Cells of Sinorhizobium meliloti WSM419 showed an adaptive acid-tolerance response when grown a t pH 58 instead of pH 70. Increasing concentrations of calcium in the exposure medium significantly decreased the death rate of WSM419 cells under conditions of acid stress (pH 4-0). The effect of calcium on survival at pH 4.0 however, appears unconnected to exopolysaccharide (EPS), since a strain with a mutation in exoY (Rm0540) responded to calcium in the exposure medium in the same way as its wild-type parent (RmZOlI). The concentration of calcium in the growth medium also affected subsequent survival a t pH 4.0, and the effect varied with pH. In cells grown at pH 58, higher calcium concentrations also markedly increased the rate of synthesis of EPS; this was not seen in cells grown a t pH 7.0. 'H NMR spectra for isolated EPS from WSM419 cultures grown a t pH 5.8 and pH 7-0 showed that low pH markedly lowered the degree of substitution with acetyl and pyruvyl groups, but not the degree of substitution with succinyl groups; calcium concentration did not affect the pattern of substitution a t either pH. For EPS to be involved in the effect of calcium concentration in the growth medium on survival would imply a deleterious effect of the EPS produced at low pH.

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