“Mark the Gene”: a Method for Nondestructive Introduction of Marker Sequences Inside the Gene Frame of Transgenes

ABSTRACT A specific marking and detection technique is a fundamental requirement for the safer use of genetically modified (GM) organisms. Here we propose a simple and effective method for directly marking functional transgenes in GM organisms. For that purpose, we introduced nucleotide substitutions (NS), based on the degeneracy of codons as markers (NS markers), into the bphC (2,3-dihydroxybiphenyl dioxygenase) and tomA3 (toluene-ortho-monooxygenase) gene frames using a PCR-based method. No change was observed in the enzyme activity of translated proteins, and alignments with homologous genes showed the uniqueness of the NS markers. Furthermore, we constructed tomA3 variations harboring NS markers in different positions. Although the translational products were identical, the constructed variation genes could be distinguished through their marker patterns by multiplex PCR, showing that NS markers could serve as product-specific tags for identifying individual GM organisms. This direct method of marking the functional transgene provides a simple, low-risk, and robust marking method without causing the gene functions to deteriorate.

Evolutionarily Divergent Extradiol Dioxygenases Possess Higher Specificities for Polychlorinated Biphenyl Metabolites

ABSTRACT The reactivities of four evolutionarily divergent extradiol dioxygenases towards mono-, di-, and trichlorinated (triCl) 2,3-dihydroxybiphenyls (DHBs) were investigated: 2,3-dihydroxybiphenyl dioxygenase (EC 1.13.11.39) from Burkholderia sp. strain LB400 (DHBDLB400), DHBDP6-I and DHBDP6-III from Rhodococcus globerulus P6, and 2,2′,3-trihydroxybiphenyl dioxygenase from Sphingomonas sp. strain RW1 (THBDRW1). The specificity of each isozyme for particular DHBs differed by up to 3 orders of magnitude. Interestingly, the K m app values of each isozyme for the tested polychlorinated DHBs were invariably lower than those of monochlorinated DHBs. Moreover, each enzyme cleaved at least one of the tested chlorinated (Cl) DHBs better than it cleaved DHB (e.g., apparent specificity constants for 3′,5′-dichlorinated [diCl] DHB were 2 to 13.4 times higher than for DHB). These results are consistent with structural data and modeling studies which indicate that the substrate-binding pocket of the DHBDs is hydrophobic and can accommodate the Cl DHBs, particularly in the distal portion of the pocket. Although the activity of DHBDP6-III was generally lower than that of the other three enzymes, six of eight tested Cl DHBs were better substrates for DHBDP6-III than was DHB. Indeed, DHBDP6-III had the highest apparent specificity for 4,3′,5′-triCl DHB and cleaved this compound better than two of the other enzymes. Of the four enzymes, THBDRW1 had the highest specificity for 2′-Cl DHB and was at least five times more resistant to inactivation by 2′-Cl DHB, consistent with the similarity between the latter and 2,2′,3-trihydroxybiphenyl. Nonetheless, THBDRW1 had the lowest specificity for 2′,6′-diCl DHB and, like the other enzymes, was unable to cleave this critical PCB metabolite (k cat app < 0.001 s−1).

Catalytic Properties of the 3-Chlorocatechol-Oxidizing 2,3-Dihydroxybiphenyl 1,2-Dioxygenase from Sphingomonas sp. Strain BN6

ABSTRACT The 2,3-dihydroxybiphenyl dioxygenase from Sphingomonassp. strain BN6 (BphC1-BN6) differs from most other extradiol dioxygenases by its ability to oxidize 3-chlorocatechol to 3-chloro-2-hydroxymuconic semialdehyde by a distal cleavage mechanism. The turnover of different substrates and the effects of various inhibitors on BphC1-BN6 were compared with those of another 2,3-dihydroxybiphenyl dioxygenase from the same strain (BphC2-BN6) as well as with those of the archetypical catechol 2,3-dioxygenase (C23O-mt2) encoded by the TOL plasmid. Cell extracts containing C23O-mt2 or BphC2-BN6 converted the relevant substrates with an almost constant rate for at least 10 min, whereas BphC1-BN6 was inactivated significantly within the first minutes during the turnover of all substrates tested. Furthermore, BphC1-BN6 was much more sensitive than the other two enzymes to inactivation by the Fe(II) ion-chelating compound o-phenanthroline. The reason for inactivation of BphC1-BN6 appeared to be the loss of the weakly bound ferrous ion, which is the cofactor in the catalytic center. A mutant enzyme of BphC1-BN6 constructed by site-directed mutagenesis showed a higher stability to inactivation by o-phenanthroline and an increased catalytic efficiency for the conversion of 2,3-dihydroxybiphenyl and 3-methylcatechol but was still inactivated during substrate oxidation.