Valinomycin precursors

Similarly, iterative NRPSs operate in a linear fashion but utilize at least one domain or module multiple times for the synthesis of a single NRP product. Thus, peptides assembled by iterative synthetases contain short, repeating units of peptide building blocks. In such systems, the terminal PCP-TE (or infrequendy PCP-C) didomain is responsible for both condensation of the repeating peptide units and chain release from the assembly line. NRPs biosynthesized in this manner include enniatin, enterobactin, bacillibactin, " gramicidin and the depsi-peptides valinomycin and cereulide. Of these examples, condensation of the precursor peptides for both enterobactin and gramicidin S has been extensively studied and will be discussed in detail. 

Studies of the incorporation of radioactice precursors into valinomycin revealed that protoplasts from Streptomyces tsusimaensis were more active than the intact mycelium, e.g. in the case of valine. 

It was interesting in the case of valinomycin, that addition of unlabelled a-ketoisovalerate to protoplasts of Streptomyces tsusi-maensis reduced the incorporation of "4c-valine into valinomycin. We suppose that a-ketoisovalerate serves as the precursor of valine as has been established for other organisms. However, more experiments will be required to prove that a-ketoisovalerate is also the precursor of D-a-hydroxyisovaleric acid. 

The Nlcotiana Fi3 is Imported into spinach mitochondria (Fig. 1, lane 2). The 59 kDa precursor is processed to a 51 kDa protein which was insensitive to externally added protease. The Neurospora Fit gives one major translation product of 56 kDa (Fig. 2, lane 1) which was imported and processed to 54 kDa (Fig. 2. lane 2). The import of the Neurospora FiP needs a membrane potential which is shown in Fig. 2. lane 4 and 5. Addition of valinomycin and oligomycin inhibits thO formation of the membrane potential, no imported products are present. 

Fig. 2. Import of the Neurospora Fi6 into spinach leaf mitochondria. Lane 1, vitro translation product. Lane 2, import. Lane 3, import followed by protease K treatment. Lane 4, import in the presence of valinomycin and oligomycin. Lane 5, import in the presence of valinomycin and oligomycin and protease K treatment. Lane 6, import, Triton X-100 treated prior to protease K treatment. Precursor of 56 kDa (p) and mature form of 54 kDa (m) indicated.

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