MALONYL-COENZYME A-ACYL CARRIER PROTEIN ACYLTRANSFERASE OF STREPTOMYCES-GLAUCESCENS - A POSSIBLE LINK BETWEEN FATTY-ACID AND POLYKETIDE BIOSYNTHESIS

Streptomyces glaucescens, a Gram-positive soil bacterium, produces the polyketide antibiotic tetracenomycin (Tcm) C. To study possible bioch emical connections between the biosynthesis of bacterial fatty acids a nd polyketides, the abundant acyl carrier protein (ACP) detected throu ghout the growth of the tetracenomycin (Tcm) C-producing S. glaucescen s was purified to homogeneity and found to behave like many other ACPs from bacteria and plants (apparent M(r) of 20 000 on gel filtration c hromatography, apparent M(r) of 3400-4800 on sodium dodecyl sulfate-po lyacrylamide gel electrophoresis under reducing conditions, and pI app roximate to 3.8). By using an oligodeoxynucleotide synthesized in acco rdance with the sequence of residues 25-36 of the ACP, the fabC gene e ncoding this protein was cloned, and expression of this gene in Escher ichia coli yielded the ACP entirely as the active holoenzyme. Sequence analysis of 4.3 kilobases (kb) of DNA flanking fabC revealed the pres ence of three other genes oriented in the same transcriptional directi on in the order fabD, fabH, fabC, and fabB. Each of the four genes is predicted to encode proteins with high sequence similarity to the foll owing components of the E. coli fatty acid synthase (FAS): the FabD ma lonyl-coenzyme A:ACP acyltransferase (MAT), FabH 3-oxoacyl:ACP synthas e III, AcpP ACP, and FabB 3-oxoacyl:ACP synthase I. Expression of the S. glaucescens fabD gene in E. coli produced active MAT able to cataly ze in vitro the transfer of radioactive malonate from malonyl-coenzyme A to the E. coli AcpP and S. glaucescens FabC ACPs, as well as to the TcmM ACP component of the Tcm type II polyketide synthase [Shen, B., et al. (1992) J. Bacteriol 174, 3818-3821]. Expression of fabD also re stored the high-temperature growth of the E. coli fabD89 mutant that b ears a temperature-sensitive MAT. The latter finding and the close sim ilarity between the organization of the S. glaucescens fabDHCB and E. coli FAS-encoding genes (fabH/fabD/fabG/acpP/fabF) suggest that the S. glaucescens genes encode FAS enzymes. Moreover, on the basis of its i n vitro activity, it is possible that the S. glaucescens FabD MAT is r esponsible for charging the TcmM ACP with malonate in vivo, a key step in the synthesis of the deca(polyketide) precursor of Tcm C. This imp lies the existence of a functional connection between fatty acid and p olyketide metabolism in this bacterium.