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When studying Streptomyces secondary metabolism under classical laboratory conditions, i.e. batch culture, secondary metabolites production begin when the culture is stressed due to depletion of a nutrient. This phenomenon is closely correlated with the transition phase - slow growth rate - of the culture and with the morphological changes that this phase implies. Because of this wide spread common observation, one role for secondary metabolites has been postulated as a mechanism by which Streptomyces can assure growth in time of famine at the expense of micro-organisms suppressed by the antibiotic activity of these secondary metabolites. Nevertheless, it may be true under laboratory conditions, it need not impliy that it is under real environmental conditions, such as those ruling in the rhizosphere.
As an alternative approach in order to contribute to the elucidation of the biological role of secondary metabolism in Streptomyces, our group suggests that under stress conditions primary metabolite conversion to secondary metabolites is a biochemical strategy through which the cell will be able to maintain energy sources and essential metabolites for a more favourable future. If this model is true, it must imply that any metabolic activity under stress must be carried out using as low a maintainence energy as possible. The rare occurence of amino acid anabolism gene regulation in Streptomyces (Hood et al., 1992) compared to well-studied enteric bacteria and Bacillus subtilis is in conformity with this principle. Our group suggests that this lack of gene regulation reflects the oligotroph life style of streptomycetes (Hu et al., 1999).
We believe that whether the secondary metabolism is tightly regulated and co-ordinated with morphological changes not induced environmentally (Hodgson, 1992), it may not be worthwhile for the cell to maintain and invest in high levels of amino acid metabolism regulation. The antibiotic activity of secondary metabolites produced by Streptomyces might be an evolution selection consequence of this phenomenon. Hitherto, there are three major exceptions to this lack of gene regulation of amino acid biosynthesis, where there is clear evidence of feed-back repression: shikimate pathway repression by tryptophan (Murphy and Katz, 1980); arginine biosynthesis (Flett et al., 1987; Soutar and Baumberg, 1996; Rodriguez-Garcia et al., 1997); and branched-chain amino acids biosynthesis (Potter and Baumberg, 1996; Craster et al., 1999).
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Of the four antibiotics produced by S. coelicolor A3(2), two use amino acids as starting material for biosynthesis. Undecylprodigiosin or Red (I) is a red pigmented non-polar tripyrrole derivative, and its biosynthesis is achieved from the precursor proline, as well as from glycine/serine and acetate residues (Wasserman et al., 1974). Our group has shown that Red can act as a natural sink in mutants with the pleiotropic effect of proline degradation and transport, suggesting a biological role for secondary metabolism (Hood et al., 1992). In the other hand, CDA (II) is a non-ribosomal lipopeptide with unusual amino acid residue including L-tryptophan and D-tryptophan (Kempter et al., 1997). Due to their role as secondary metabolite precursors, tryptophan and proline metabolism in S. coelicolor A3(2) has been studied in our laboratory. Originally, these two amino acids were chosen because in S. coelicolor A3(2) proline appeared to be a secondary metabolite precursor, while tryptophan was not known to have this role. This strategy could allow a comparative analysis of metabolism regulation of both amino acids within the same organism. Nevertheless, until very recently the CDA chemical structure has been elucidated (Kempter et al., 1997), showing that its biosynthesis uses tryptophan as a precursor. Although this discovery nullifies our original strategy as it was conceived, the study of more then one pathway will be useful to gain integrative insights into the metabolic regulation of primary and secondary metabolism. |
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R1 can be OPO3H2 or OH; R2 can be H or CH3 Residues with D-configuration are indicated by D |
Our group has demonstrated
that in S. coelicolor A3(2), the expression of trpC and
trpBA genes is regulated by growth rate and growth phase but
not by feedback repression. Parallel to this discovery, the trpD
gene was shown to be growth phase dependent and not regulated by
feedback repression (Hu et al.,
1999). These
results are in conformity with the lack of genetic regulation of
amino acid biosynthesis in Streptomyces. Compared to E.
coli and B. subtilis, where tryptophan biosynthesis has
become a classical example of feedback gene regulation (Pittard,
1996; Landick et al., 1996; and Babitzke, 1997) tryptophan
biosynthesis regulation in S. coelicolor A3(2) seems to be
very different, and just recently has received increased attention.
In S. coelicolor A3(2) the trp genes are not
clustered together, but instead they are arranged in three different
loci: trpA, trpB and trpC map to a single locus
at about 11 o'clock; trpD and trpF map to locus at
about 10 o'clock; and trpEG map to about half past ten
(Smithers and Engels, 1974; Redenbach et
al., 1996). Past work in our group has lead to the cloning of
all the trp genes of S. coelicolor A3(2), except for
trpF gene. The latter was reveal not to be clustered with the
trpD gene after DNA sequence determination. Considering this,
and the fact that the only trpF mutant available isolated by
Smithers and Engles (1974) has been lost,
it is unclear where the trpF gene is physically located.
Furthermore, in the same piece of research were identified the
transcription start sites of trpC and trpXBA. DNA
sequence reveals that trpX is an open reading frame with no
homology with anything from the database, and may have a regulatory
role since it is co-transcribed with the trpBA genes (Hu,
1995; Hu et al., 1999).
Additionally, our group has identified from DNA sequence analysis a putative classical attenuator upstream the trpEG genes (Hu, 1995). Lin et al. (1998) have reported also a putative attenuator in front of the trpEG genes of S. venezuelae. However, it seems that for gene regulation studies in Streptomyces, predictions based in DNA sequence are not always reliable. Similar classical attenuation structures have been identified upstream the ilvB and leuA genes of the branched-chain amino acids biosynthetic pathway in S. coelicolor A3(2), but site-directed mutagenesis in such structures had no effect in the gene regulation and the wild-type phenotype persisted (Craster et al., 1999)
