Benzene is one of the most toxic and prevailing environmental pollutants. Rhodococcus sp. 33 can tolerate and efficiently degrade various concentrations of benzene. Under either resting or growth conditions, rough mutant strains derived from strain 33 were more sensitive to benzene than the mucoidal parent strain. The rough strains did not produce extracellular polysaccharides (EPS), whereas the parental strain 33 did so in large quantity (33 EPS). By the addition of 33 EPS to the rough strains, both the survival and growth of the rough strains in media containing benzene were improved. The 33 EPS was found to be an acidic polysaccharide containing D-galactose, D-glucose, D-mannose, D-glucuronic acid, and pyruvic acid at a molar ratio of 1: 1: 1: 1: 1. These data suggest that the 33 EPS plays an important role in the benzene tolerance in Rhodococcus sp. 33, especially helping the cells to survive an initial challenge with benzene.
Infection of a Streptomyces galbus strain R-5 in leaves of tissue-cultured seedlings of rhododendron was examined using transmission electron microscopy and a histochemical method. Mycelia invaded the host through stomatal openings but not through other sites. Mycelia colonizing leaf surfaces and those growing through stomatal openings were embedded in an electron-dense, thick, amorphous material, while single hyphae were not. Within host tissues, hyphae were observed individually or in colonies in intercellular spaces but not in epidermal or mesophyll cells. The mycelia colonizing in intercellular spaces were also embedded in a similar amorphous material. Wall appositions formed within the epidermal and mesophyll cells, the outer surfaces of which were attached to the intercellular mycelia, suggesting that those cells had recognized the presence of mycelia of R-5 and responded to it. On the leaf surfaces, the wax that covered the undulating cuticular layer was degraded beneath the growing mycelia. This observation was supported by the presence of indigo blue crystals that formed as hydrolytic products of mycelial nonspecific esterase along the mycelia on fresh leaves.
Tissue-cultured seedlings of mountain laurel (Kalmia latifolia L.) were treated in flasks with strain AOK-30 of Streptomyces padanus. In a previous report, this treatment induced drought tolerance in the seedlings. Because structural modification of cell walls and enhancement of osmotic pressure in the cells were the likely cause, cell walls of AOK-30-treated and -untreated seedlings were further analyzed focusing on callose accumulation and lignification. About 2.5 times more callose accumulated in AOK-30-treated than -untreated seedlings. Sequence analysis of amino acids derived from cell wall proteins proved that cell walls of treated seedlings contained an enzyme with 100% homology to putative mitochondrial NAD-dependent malate dehydrogenase in potato. Malate dehydrogenase has been known to bind the apoplast and cell walls of other plants and is thought to be associated with lignification. Histochemical observations using phloroglucine-HCl revealed that cell walls of sieve cells were more intensely lignified in AOK-30-treated seedlings in contrast to the weak staining in untreated seedlings. Based on these results, the drought tolerance of K. latifolia seedlings induced by AOK-30 treatment was shown to be associated with accelerated callose accumulation and lignification in cell walls of sieve cells.
Nitrile can be degraded by nitrile hydratase (NHase) to the corresponding amide, which is then converted to the acid plus ammonium by amidase. Nitrile can also be directly hydrolyzed to the corresponding acid plus ammonium by nitrilase. NHases and nitrilases are widely distributed among genus Rhodococcus, and two types of NHases (Fe-type and Co-type) and versatile nitrilases have been isolated from several Rhodococcus species. NHases are composed of α- and β-subunits, which differ in size from each other, and two kinds of Co-type NHases, high-molecular-mass (H-NHase) and low-molecular-mass (L-NHase) ones, have been discovered in R. rhodochrous J1. Characterization of their enzymes at the molecular level has provided new insights into how the molecular structures determine these enzyme functions, and how the regulatory systems control the expression of the enzyme genes to improve the enzymes. The practical use of Rhodococcus in biotechnology, and the application of Rhodococcus NHase and nitrilase for the industrial production of useful compounds such as acrylamide, nicotinamide and several vitamins have been recognized and developed.