A novel aerobic pentachloronitrobenzene-degrading bacterium, Nocardioides sp. strain PD653, was isolated from an enrichment culture in a soil-charcoal perfusion system. The bacterium also degraded hexachlorobenzene, a highly recalcitrant environmental pollutant, accompanying the generation of chloride ions. Liberation of 14 CO 2 from [U-ring-14 C]hexachlorobenzene was detected in a culture of the bacterium and indicates that strain PD653 is able to mineralize hexachlorobenzene under aerobic conditions. The metabolic pathway of hexachlorobenzene is initiated by oxidative dechlorination to produce pentachlorophenol. As further intermediate metabolites, tetrachlorohydroquinone and 2,6-dichlorohydroquinone have been detected. Strain PD653 is the first naturally occurring aerobic bacteria capable of mineralizing hexachlorobenzene.

Nocardioides sp. strain MTD22 degraded atrazine, ametryn and atraton, as did Arthrobacter aurescens strain TC1 and Nocardioides sp. strain C190. These strains contain trzN, a gene coding for TrzN, triazine hydrolase showing a broad substrate range. However, Nocardioides sp. strain AN3 degraded only atrazine despite containing trzN. These differences in s-triazine degradation are presumed to be due to differences in the amino acid sequences of TrzNs. Consequently, 1371 nucleotides of the trzN coding sequences of strains AN3 and MTD22 were determined. Comparisons of the amino acid sequences of TrzNs indicated that three residues of strain AN3 (Thr(214), His(215) and Gln(241)) were distinct from those of the other three strains (Pro(214), Tyr(215) and Glu(241)). To confirm the relationships between these amino acid sequences and the substrate specificities of TrzNs, wild and chimera trzN genes were constructed and expressed in Escherichia coli cells. Cells expressing wild MTD22 trzN (Pro(214)Tyr(215)Glu(241)) and chimera AN3-MTD22 trzN (Thr(214)His(215)Glu(241)) degraded all s-triazines, but the degradation rate was markedly decreased in AN3-MTD22 trzN. Wild AN3 trzN (Thr(214)His(215)Gln(241)) and chimera MTD22-AN3 trzN (Pro(214)Tyr(215)Gln(241)) degraded only atrazine. These results suggest that the substitution of Glu(241) for Gln(241) significantly decreases enzyme affinity for ametryn and atraton.

A Gram-positive bacterial strain able to degrade the herbicide atrazine was isolated using a simple model ecosystem constituted with Japanese riverbed sediment and its associated water (microcosm). Treatment of the water phase of the microcosm with 1 mg litre-1 [ring-14C]atrazine resulted in the rapid degradation of atrazine after a 10 day lag phase period. The [ring-14C]cyanuric acid formed was transiently accumulated as an intermediary metabolite in the water phase and was subsequently mineralised through triazine ring cleavage. Possible atrazine-degrading microbes suspended in the water phase of the microcosm were isolated by the plating method while rapid degradation of atrazine was in progress. Among the 48 strains that were isolated, 47 exhibited atrazine-degrading activity. From these 47 isolates, 12 strains that were randomly selected were found to identically convert atrazine to cyanuric acid via hydroxyatrazine. Polymerase chain reaction (PCR) amplification of the genes corresponding to atrazine degradation revealed that these strains at least carried the genes trzN (atrazine chlorohydrolase from Nocardioides C190) and atzC (N-isopropylammelide isopropyl amidohydrolase from Pseudomonas ADP). Physiological characteristics and 16S rDNA partial sequences of six strains that were further selected strongly suggested that all these isolates originated from the same Nocardioides sp. strain. Additionally, only one isolate could mineralise the triazine ring of cyanuric acid. Based on microscopic observations, this strain appears to be a two-membered microbial consortium consisting of Nocardioides sp. and a Gram-negative bacterium. In conclusion, atrazine biodegradation in the microcosm appeared to occur predominantly by Nocardioides sp. to yield cyanuric acid, which could be mineralised by the other relatively ubiquitous microbes.

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Although strain RA8 can grow on linuron, some elements in the R2A broth seemed significantly to stimulate its growth and ability to degrade. The isolate strictly recognised the structural difference between N-methoxy-N-methyl and N,N-dimethyl substitution of various phenylurea herbicides.