Pmt gene

Once the supply of putrescine was enhanced, no larger increases in nicotine were found, presumably because other enzymes contributed, more than previously, to limiting nicotine accumulation. MPO is present at, t) ically, two-to fivefold higher levels than PMT, and therefore PMT may become limiting. Now that the pmt gene has been cloned (Hibi et al., 1994), this possibility can be tested directly. 

Nevertheless, genetic engineering of a key enzyme in a chosen pathway does not always result in the enhancement of the end product thus, overexpression of pmt gene carried out in A. belladonna resulted in a 5-fold increased in pmt transcript level but an unchanged tropane alkaloid profile (hyoscyamine and scopolamine), as well as the tropane alkaloids biosynthetic precursors tropine, pseudotropine and tropinone which were not affected [134-135], Similarly, the overexpression of tobacco pmt gene in Dubosia hybrid hairy root cultures produced amounts of tropane alkaloid similar to control roots [131, 136]. All these results seem to indicate the presence of different points of control in the tropane alkaloid metabolic pathway. 

Gentzsch, M., and Tanner, W. (1996). The PMT gene family Protein O-glycosylation in Saccharomyces cerevisiae is vital. EMBO J. 15, 5752—5759. 

The use of undifferentiated cultures proved to be unsuccessful for tropane alkaloids production. These alkaloids are produced in normal and transformed roots [42, 51]. Several lines of evidence suggest that the differentiation of the tissue is necessary for the synthesis of these metabolites [42, 51]. Different studies suggest that this is probably related to the localization of key biosynthetic enzymes [6]. Among them, Suzuki et al. [52, 53] demonstrated that the h6h and pmt genes were expressed specifically in root pericycle of Atropa belladonna plants. In addition, Nakajima et al. [54, 55] pointed out that Tropinone reductase enzymes were accumulated in lateral roots of Hyoscyamus niger. 

Moyano, E., Fomale, S., Palazon, J., Cusido, R. M., Bagni, N., and Pinol, M. T. (2002) Alkaloid production in Duboisia hybrid hairy root cultures overexpressing the pmt gene. Phytochemistry. 59, 697-702. 

Moyano E, Jouhikainen K, Tammela P, Palazon J, Cusido RM, Pinol MT, Teeri TH, Oksman-Caldentey KM (2003) Effect of pmt gene overexpression on tropane alkaloid production in transformed root cultures of Datura metel and Hyoscyamus muticus. J Exp Bot 54(381) 203-211. doi 10.1093/jxb/erg014... 

Studies using synthetic peptides indicate that threonine is a better mannosyl acceptor in vitro, but the in vivo acceptor preferences of the individual Pmtps are not yet known. The existence of multiple Pmtps with differing acceptor specificities, coupled with the potential for these proteins to form heterodimers, may permit a wider range of peptide sequences to be recognized as substrates. However, deletion experiments with yeast PMT genes show that in some cases, a given Pmtp may be predominantly responsible for G-mannosylation of an individual protein, but that in others, several Pmtps have overlapping functions in G-mannosylation of a protein. 

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