INITIATION OF BIOSYNTHESIS IN CIS POLYISOPRENES

The molecular species initiating rubber biosynthesis in leaves of Soli dago altissima and Helianthus annuus and sporophores of Lactarius vole mus have been analysed by structural characterization of the polyisopr enes by C-13 NMR spectroscopy. The alignment of trans and cis isoprene units in these cis polyisoprenes has been determined using lyprenol-1 6[dimethylallyl-(trans)(2)-(cis)(13)-OH] and ficaprenol-11 [dimethylal lyl-(trans)(3)-(cis)(7)-OH], respectively, as model compounds. The C-1 methylene carbon atoms of the trans isoprene units give rise to two a nd three signals at delta 39.7-39.8 for polyprenol-16 and ficaprenol-1 1, respectively. These signals were assigned by comparison with the si gnal splitting pattern in solanesol-9 and spin-lattice relaxation time s T-1 of the respective carbon atoms, also. Polyprenol-16 gives rise t o a signal reflecting the presence of the trans unit in the trans-tran s-trans sequence at about 5%. Similarly, ficaprenol-11 contains about 5% of the trans-tl ans sequence. Rubbers from S. altissima and H. annu us give rise to trans C-1 carbon signals, indicative of the presence o f both two trans and three trans sequences in a ratio of 4:3 and 2:1, respectively. By contrast, only the dimethylallyl-(trans)(2)-sequence was observed in the low M(r) cis polyisoprene from sporophores of L. v olemus. These findings demonstrate that the initiating species of rubb er formation in the leaves are both trans,trans-farnesyl diphosphate ( FDP) and trans,trans,trans-geranylgeranyl diphosphate (GGDP). Taking i nto account the presence of about 5% of the unexpected isomeric termin al sequences in these polyprenols, it is concluded that the primer sel ectivity of rubber transferase in the leaves is not highly specific wi th respect to the chain length of the allylic diphosphate initiator.