The nucleotide sequences of the 27,939-bp-long upstream and 9,448-bp-long downstream regions of the carAaAaBaBbCAc(ORF7)Ad genes of carbazole-degrading Pseudomonas sp. strain CA10 were determined. Thirty-two open reading frames (ORFs) were identified, and the car gene cluster was consequently revealed to consist of 10 genes (carAaAaBaBbCAcAdDFE) encoding the enzymes for the three-step conversion of carbazole to anthranilate and the degradation of 2-hydroxypenta-2,4-dienoate. The high identities (68 to 83%) with the enzymes involved in 3-(3-hydroxyphenyl)propionic acid degradation were observed only for CarFE. This observation, together with the fact that two ORFs are inserted between carD and carFE, makes it quite likely that the carFE genes were recruited from another locus. In the 21-kb region upstream from carAa, aromaticring-hydroxylating dioxygenase genes (ORF26, ORF27, and ORF28) were found. Inductive expression in carbazole-grown cells and the results of homology searching indicate that these genes encode the anthranilate 1,2-dioxygenase involved in carbazole degradation. Therefore, these ORFs were designated antABC. Four homologous insertion sequences, IS5car1 to IS5car4, were identified in the neighboring regions of car and ant genes. IS5car2 and IS5car3 constituted the putative composite transposon containing antABC. One-ended transposition of IS5car2 together with the 5 portion of antA into the region immediately upstream of carAa had resulted in the formation of IS5car1 and ORF9. In addition to the insertion sequence-dependent recombination, gene duplications and presumed gene fusion were observed. In conclusion, through the above gene rearrangement, the novel genetic structure of the car gene cluster has been constructed. In addition, it was also revealed that the car and ant gene clusters are located on the megaplasmid pCAR1.

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Several recent studies have suggested that the reactive oxygen species (ROS) generated from mitochondria contribute to genomic instability after exposure of the cells to ionizing radiation, but the mechanism of this process is not yet fully understood. We examined the hypothesis that irradiation induces mitochondrial dysfunction to cause persistent oxidative stress, which contributes to genomic instability. After the exposure of cells to 5 Gy gamma-ray irradiation, we found that the irradiation induced the following changes in a clear pattern of time courses. First, a robust increase of intracellular ROS levels occurred within minutes, but the intracellular ROS disappeared within 30 minutes. Then the mitochondrial dysfunction was detected at 12 hours after irradiation, as indicated by the decreased activity of NADH dehydrogenase (Complex I), the most important enzyme in regulating the release of ROS from the mitochondrial electron transport chain (ETC). Finally, a significant increase of ROS levels in the mitochondria and the oxidation of mitochondrial DNA were observed in cells at 24 hours or later after irradiation.Although further experiments are required, results in this study support the hypothesis that mitochondrial dysfunction causes persistent oxidative stress that may contribute to promote radiation-induced genomic instability.2

The carbazole 1,9a-dioxygenase (CARDO) system of Pseudomonas resinovorans strain CA10 consists of terminal oxygenase (CarAa), ferredoxin (CarAc), and ferredoxin reductase (CarAd). Each component of CARDO was expressed in Escherichia coli strain BL21(DE3) as a native form (CarAa) or a His-tagged form (CarAc and CarAd) and was purified to apparent homogeneity. CarAa was found to be trimeric and to have one Rieske type [2Fe-2S] cluster and one mononuclear iron center in each monomer. Both His-tagged proteins were found to be monomeric and to contain the prosthetic groups predicted from the deduced amino acid sequence (His-tagged CarAd, one FAD and one [2Fe-2S] cluster per monomer protein; His-tagged CarAc, one Rieske type [2Fe-2S] cluster per monomer protein). Both NADH and NADPH were effective as electron donors for His-tagged CarAd. However, since the k cat /K m for NADH is 22.3-fold higher than that for NADPH in the 2,6-dichlorophenolindophenol reductase assay, NADH was supposed to be the physiological electron donor of CarAd. In the presence of NADH, His-tagged CarAc was reduced by His-tagged CarAd. Similarly, CarAa was reduced by His-tagged CarAc, His-tagged CarAd, and NADH. The three purified proteins could reconstitute the CARDO activity in vitro. In the reconstituted CARDO system, His-tagged CarAc seemed to be indispensable for electron transport, while His-tagged CarAd could be replaced by some unrelated reductases.Pseudomonas resinovorans strain CA10 is a bacterium that has the ability to utilize carbazole (CAR) as its sole source of carbon, nitrogen, and energy. As the initial degradation reaction, CAR is dioxygenated at the angular (C-9a) and adjacent (C-1) positions to yield the unstable cis-hydrodiol (25, 27). The resultant cis-hydrodiol is spontaneously converted to 2Ј-aminobiphenyl-2,3-diol, which is further converted to anthranilate via meta cleavage and hydrolysis. Such initial dioxygenation is called angular dioxygenation and contains the degradation pathways of CAR and its structural analogues (25).Genes encoding CAR 1,9a-dioxygenase (CARDO) were cloned from P. resinovorans strain CA10, and CARDO was found to be a multicomponent enzyme system which consists of terminal oxygenase (CarAa), ferredoxin (CarAc), and ferredoxin reductase (CarAd) (32). Amino acid sequence homology and phylogenetic analysis revealed that CarAa is a rather unique type of oxygenase that shares low homology with other known dioxygenases (23, 32). Furthermore, from studies on substrate specificity, it was found that CARDO has a broad substrate range and can catalyze diverse oxygenations: angular dioxygenation, lateral dioxygenation, and monooxygenation (26). In addition, Habe et al. (11) have reported that CARDO can attack some chlorinated dioxin congeners in a manner similar to that of dibenzofuran 4,4a-dioxygenase from Terrabacter sp. strain DBF63 and dioxin dioxygenase from Sphingomonas wittichii strain RW1 (43), although some difference was observed in particular features. Compared to other degradative reactions for dio...