Phosphorylation of the AfsR Protein Involved in Secondary Metabolism in Streptomyces Species by a Eukaryotic-type Protein Kinase

A global regulatory protein, AfsR, involved in secondary metabolism, was found to be phosphorylated by a membrane-associated phosphokinase, named AfsK, of Streptomyces coelicolor A3(2) and S. lividans. The N-terminal portion of AfsK, deduced from the nucleotide (nt) sequence of the afsK gene, which was located downstream from the afsR gene, showed significant sequence similarity to the catalytic domain of eukaryotic Ser/Thr protein kinases (PKs). Consistent with this, experiments with AfsK produced by use of an Escherichia coli host-vector system revealed a self-catalyzed phosphate incorporation into both Ser and Tyr residues of AfsK. The recombinant AfsK phosphorylated the purified AfsR at both Ser and Thr residues. Disruption of the chromosomal afsK gene with the phage vector KC515 resulted in significant, but not complete, loss of actinorhodin production. This result implies the involvement of afsK in the regulation of secondary metabolism. The presence of an additional PK able to phosphorylate AfsR is predicted, because the afsK-disrupted strain still contained an activity able to phosphorylate Ser and Thr residues of AfsR. Southern hybridization experiments showed that nt sequences homologous to afsK, as well as afsR, were distributed among many Streptomyces spp. It is thus concluded that a signal transduction system similar to that found in higher organisms is involved in the regulation of secondary metabolism in the bacterial genus Streptomyces.