Ectopic expression of the Streptomyces coelicolor whiE genes for polyketide spore pigment synthesis and their interaction with the act genes for actinorhodin biosynthesis

Abstract

The whiE gene cluster of Streptomyces coelicolor is normally expressed shortly before sporulation in the aerial mycelium, leading to production of the grey polyketide spore pigment. By placing the whiE genes under the control of the thiostrepton-inducible tipA promoter, they were artificially expressed on plasmids or in the chromosome during vegetative growth in a strain deleted for the act genes, which control biosynthesis of the polyketide antibiotic actinorhodin. Certain combinations of whiE-ORFI-VII led to production of mycelial pigments; these were exported into the medium when whiE-ORFI was absent, but poorly in its presence. Combined with comparative sequence data, the results allowed deductions to be made, or confirmed, about the normal roles of the eight known genes, whiE-ORFI-VIII, as follows: whiE-ORFIII, IV, V encode the three components (ketosynthase, chain length factor and acyl carrier protein) of the whiE 'minimal' polyketide synthase (PKS) needed for assembly of the carbon chain of the spore pigment precursor; whiE-ORFII, VI, VII are likely to be involved in cyclizations of the nascent carbon chain; whiE-ORFVIII controls a late step in the spore pigment biosynthetic pathway, probably a hydroxylation; and whiE-ORFI may encode a protein needed for correct targeting or retention of spore pigment at an appropriate cellular location. In other experiments, genes encoding components of the act-PKS and whiE-PKS were artificially co-expressed. Each of the three whiE minimal PKS subunit genes could complement lesions in the corresponding act-PKS genes to produce actinorhodin or related mycelial pigments, and each of the three act minimal PKS genes could complement lesions in the whiE minimal PKS genes to cause spore pigmentation. Thus the two sets of PKS subunits, which are encoded by genes that have presumably diverged from a common ancestor, are still capable of biochemical 'cross-talk', but this is normally prevented because the gene sets are expressed in different 'tissues' of the differentiated Streptomyces colony. Ectopic expression of sets of whiE-PKS genes presumed to be sufficient to assemble a carbon chain caused inhibition of early growth of the strains, perhaps by causing interference with fatty acid biosynthesis; this yielded circumstantial evidence that the whiE-PKS gene products can also interact with those of the fatty acid synthase(s) of the organism.