Conjugal transfer of recombinant plasmids into gellan gum-producing and non-producing variants of Pseudomonas elodea ATCC 31461

Cloning and characterization of genes involved in nostoxanthin biosynthesis of Sphingomonas elodea ATCC 31461

Most Sphingomonas species synthesize the yellow carotenoid nostoxanthin. However, the carotenoid biosynthetic pathway of these species remains unclear. In this study, we cloned and characterized a carotenoid biosynthesis gene cluster containing four carotenogenic genes (crtG, crtY, crtI and crtB) and a β-carotene hydroxylase gene (crtZ) located outside the cluster, from the gellan-gum producing bacterium Sphingomonas elodea ATCC 31461. Each of these genes was inactivated, and the biochemical function of each gene was confirmed based on chromatographic and spectroscopic analysis of the intermediates accumulated in the knockout mutants. Moreover, the crtG gene encoding the 2,2′-β-hydroxylase and the crtZ gene encoding the β-carotene hydroxylase, both responsible for hydroxylation of β-carotene, were confirmed by complementation studies using Escherichia coli producing different carotenoids. Expression of crtG in zeaxanthin and β-carotene accumulating E. coli cells resulted in the formation of nostoxanthin and 2,2′-dihydroxy-β-carotene, respectively. Based on these results, a biochemical pathway for synthesis of nostoxanthin in S. elodea ATCC 31461 is proposed.

Identification of the pgmG gene, encoding a bifunctional protein with phosphoglucomutase and phosphomannomutase activities, in the gellan gum-producing strain Sphingomonas paucimobilis ATCC 31461

The pgmG gene of Sphingomonas paucimobilis ATCC 31461, the industrial gellan gum-producing strain, was cloned and sequenced. It encodes a 50,059-Da polypeptide that has phosphoglucomutase (PGM) and phosphomannomutase (PMM) activities and is 37 to 59% identical to other bifunctional proteins with PGM and PMM activities from gram-negative species, including Pseudomonas aeruginosa AlgC. Purified PgmG protein showed a marked preference for glucose-1-phosphate (G1P); the catalytic efficiency was about 50-fold higher for G1P than it was for mannose-1-phosphate (M1P). The estimated apparent K(m) values for G1P and M1P were high, 0.33 and 1.27 mM, respectively. The pgmG gene allowed the recovery of alginate biosynthetic ability in a P. aeruginosa mutant with a defective algC gene. This result indicates that PgmG protein can convert mannose-6-phosphate into M1P in the initial steps of alginate biosynthesis and, together with other results, suggests that PgmG may convert glucose-6-phosphate into G1P in the gellan pathway.

Emergence of Cu(++)-tolerant mutants defective in gellan synthesis in Cu(++)-stressed cultures of Sphingomonas paucimobilis

Cells defective in gellan synthesis appeared during cultivation of the gellan gum-producing strain Sphingomonas paucimobilis R40 with inhibitory concentrations of copper, supplied as CuCl2. The percentage of less mucoid colonial variants dramatically increased with the increase in Cu++ supplementation, reaching 85% of total viable cells at the maximal concentration for growth. Results reported in this work indicate that emergence of colonial variants defective in gellan synthesis results from Cu(++)-induced mutation and the growth advantage of these mutants in Cu(++)-stressed cultures. In fact, DNA homologous recombination strongly increased with the increase in copper supplementation as indicated by the regeneration of kanamycin-resistant cells of R40 harbouring plasmid pBX404-7, which carries two non-overlapping truncated genes derived from a gene conferring kanamycin resistance. The four major groups of colonial mutants that emerged from Cu(++)-stressed cultures of R40 exhibited reduced growth rate and biomass yield in the absence of Cu++ stress and produced decreased levels of exopolysac-charide (EPS) which yielded solutions of lower or negligible viscosity. The level of increased Cu++ tolerance of these mutants, assessed by the inhibitory effect of Cu++ on growth, correlated with the degree of loss of the ability to secrete high-molecular-mass EPS. Consistent with the growth advantage of gellan-defective mutants in Cu(++)-stressed cultures, the non-producing strain RP10, spontaneously obtained during extended cultivation of R40, also exhibited a higher tolerance to Cu++. In addition, its non-mucoid phenotype was stably maintained during Cu(++)-stressed cultivation despite the stimulation of homologous recombination by Cu++.