Hairy root transformation of Brassica rapa with bacterial halogenase genes and regeneration to adult plants to modify production of indolic compounds.

During the last years halogenated compounds have drawn a lot of attention. Metabolites with one or more halogen atoms are often more active than their non-halogenated derivatives like indole-3-acetic acid (IAA) and 4-Cl-IAA. Within this work, bacterial flavin-dependent tryptophan halogenase genes were inserted into Brassica rapa ssp. pekinensis (Chinese cabbage) with the aim to produce novel halogenated indole compounds. It was investigated which tryptophan-derived indole metabolites, such as indole glucosinolates or potential degradation products can be synthesized by the transgenic root cultures. In vivo and in vitro activity of halogenases heterologously produced was shown and the production of chlorinated tryptophan in transgenic root lines was confirmed. Furthermore, chlorinated indole-3-acetonitrile (Cl-IAN) was detected. Other tryptophan-derived indole metabolites, such as IAA or indole glucosinolates were not found in the transgenic roots in a chlorinated form. The influence of altered growth conditions on the amount of produced chlorinated compounds was evaluated. We found an increase in Cl-IAN production at low temperatures (8 °C), but otherwise no significant changes were observed. Furthermore, we were able to regenerate the wild type and transgenic root cultures to adult plants, of which the latter still produced chlorinated metabolites. Therefore, we conclude that the genetic information had been stably integrated. The transgenic plants showed a slightly altered phenotype compared to plants grown from seeds since they also still expressed the rol genes. By this approach we were able to generate various stably transformed plant materials from which it was possible to isolate chlorinated tryptophan and Cl-IAN.

Characterization of the Aminotransferase ThdN from Thienodolin Biosynthesis in Streptomyces albogriseolus.

In Streptomyces albogriseolus the indolethiophen alkaloid thienodolin is derived from tryptophan. The first step in thienodolin biosynthesis is the regioselective chlorination of tryptophan in the 6-position of the indole ring. The second step is catalyzed by the aminotransferase ThdN. ThdN shows sequence homology (up to 69 % similarity) with known pyridoxal 5'-phosphate-dependent aminotransferases of the aspartate aminotransferase family from Gram-positive bacteria. thdN was heterologously expressed in Pseudomonas fluorescens, and the enzyme was purified by nickel-affinity chromatography. ThdN is a homodimeric enzyme with a mass of 90 600 kDa and catalyzes the conversion of l-tryptophan and a number of chlorinated and brominated l-tryptophans. The lowest KM values were found for 6-bromo- and 6-chlorotryptophan (40 and 66 µm, respectively). For l-tryptophan it was 454 µm, which explains why thienodolin is the major product and dechlorothienodolin is only a minor component. The turnover number (kcat ) for 7-chlorotryptophan (128 min-1 ) was higher than that for the natural substrate 6-chlorotryptophan (88 min-1 ).

A tryptophan 6-halogenase and an amidotransferase are involved in thienodolin biosynthesis.

The biosynthetic gene cluster for the plant growth-regulating compound thienodolin was identified in and cloned from the producer organism Streptomyces albogriseolus MJ286-76F7. Sequence analysis of a 27 kb DNA region revealed the presence of 21 ORFs, 14 of which are involved in thienodolin biosynthesis. Three insertional inactivation mutants were generated in the sequenced region to analyze their involvement in thienodolin biosynthesis and to functionally characterize specific genes. The gene inactivation experiments together with enzyme assays with enzymes obtained by heterologous expression and feeding studies showed that the first step in thienodolin biosynthesis is catalyzed by a tryptophan 6-halogenase and that the last step is the formation of a carboxylic amide group catalyzed by an amidotransferase. The results led to a hypothetical model for thienodolin biosynthesis.

Flavin-dependent halogenases involved in secondary metabolism in bacteria.

The understanding of biological halogenation has increased during the last few years. While haloperoxidases were the only halogenating enzymes known until 1997, it is now clear that haloperoxidases are hardly, if at all, involved in biosynthesis of more complex halogenated compounds in microorganisms. A novel type of halogenating enzymes, flavin-dependent halogenases, has been identified as a major player in the introduction of chloride and bromide into activated organic molecules. Flavin-dependent halogenases require the activity of a flavin reductase for the production of reduced flavin, required by the actual halogenase. A number of flavin-dependent tryptophan halogenases have been investigated in some detail, and the first three-dimensional structure of a member of this enzyme subfamily, tryptophan 7-halogenase, has been elucidated. This structure suggests a mechanism involving the formation of hypohalous acid, which is used inside the enzyme for regioselective halogenation of the respective substrate. The introduction of halogen atoms into non-activated alkyl groups is catalysed by non-heme FeII alpha-ketoglutarate- and O2-dependent halogenases. Examples for the use of flavin-dependent halogenases for the formation of novel halogenated compounds in in vitro and in vivo reactions promise a bright future for the application of biological halogenation reactions.

Digitoxosyltetracenomycin C and glucosyltetracenomycin C, two novel elloramycin analogues obtained by exploring the sugar donor substrate specificity of glycosyltransferase ElmGT.

Our explorations of glycosyltransferase ElmGT from Streptomyces olivaceus Tü 2353, which shows an interesting flexibility regarding its sugar donor substrate, were extended toward various previously unexplored sugar co-substrates. The studies revealed that ElmGT, which normally transfers L-rhamnose to 8-demethyltetracenomycin C as a crucial biosynthetic step in elloramycin biosynthesis, is also able to process an activated non-deoxygenated sugar, NDP-D-glucose, as well as NDP-L-digitoxose, which is the first example of an NDP-L-sugar co-substrate of ElmGT possessing an axial 3-OH group. The structures of the resulting novel elloramycin analogues of these experiments, 8-demethyl-8-L-digitoxosyltetracenomycin C (4) and 8-demethyl-8-D-glucosyltetracenomycin C (7), were elucidated mainly by (1)H and (13)C NMR spectroscopy and by mass spectrometry.