DNA encoding methymycin and pikromycin

#Polyhydroxyalkanoates (PHAs) are one class of biodegradable polymers. The first identified member of the PHAs thermoplastics was polyhydroxybutyrate (PHB), the polymeric ester of D(−)-3-hydroxybutyrate. The biosynthetic pathway of PHB in the gram negative bacterium Alcaligenes eutrophus is depicted in FIG. 1. PHAs related to PHB differ in the structure of the pendant arm, R (FIG. 2). For example, R═CH3 in PHB, while R═CH2CH3 in polyhydroxyvalerate, and R═(CH2)4CH3 in polyhydroxyoctanoate.

#The genes responsible for PHB synthesis in A. eutrophus have been cloned and sequenced. (Peoples et al., J. Biol. Chem., 264, 15293 (1989); Peoples et al., J. Biol. Chem., 264, 15298 (1989)). Three enzymes: β-ketothiolase (phbA), acetoacetyl-CoA reductase (phbB), and PHB synthase (phbC) are involved in the conversion of acetyl-CoA to PHB. The PHB synthase gene encodes a protein of Mr=63,900 which is active when introduced into E. coli (Peoples et al., J. Biol. Chem., 264, 15298 (1989)).

#Although PHB represents the archetypical form of a biodegradable thermoplastic, its physical properties preclude significant use of the homopolymer form. Pure PHB is highly crystalline and, thus, very brittle. However, unique physical properties resulting form the structural characteristics of the R groups in a PHA copolymer may result in a polymer with more desirable characteristics. These characteristics include altered crystallinity, UV weathering resistance, glass to rubber transition temperature (Tg), melting temperature of the crystalline phase, rigidity and durability (Holmes et al., EPO 00052 459; Anderson et al., Microbiol. Rev., 54, 450 (1990)). Thus, these polyesters behave as thermoplastics, with melting temperatures of 50-180° C., which can be processed by conventional extension and molding equipment.

#FIG. 1. The PHB biosynthetic pathway in A. eutrophus.

#FIG. 2. Molecular structure of common bacterial PHAs. Most of the known PHAs are polymers of 3-hydroxy acids possessing the general formula shown. For example, R═CH3 in PHB, T═CH2CH3 in polyhydroxyvalerate (PHV), and R═(CH2)4CH3 in polyhydroxyoctanoate (PHO).

#FIG. 3. Comparison of the natural and recombinant pathways for PHB synthesis. The three enzymatic steps of PHB synthesis in bacteria involving 3-ketothiolase, acetoacetyl-CoA reductase, and PHB synthase are shown on the left. The two enzymatic steps involved in PHB synthesis in the pathway in Sf21 cells containing a rat fatty acid synthase with an inactivated dehydrase domain (ratF AS206) are shown on the right.

#The invention described herein can be used for the production of a diverse range of biodegradable PHA polymers through genetic redesign of DNA encoding a FAS or a PKS such as that found in Streptomyces spp. Type I PKS polypeptide to provide a recombinant PHA monomer synthase. Different PHA synthases can then be tested for their ability to polymerize the monomers produced by the recombinant PHA synthase into a biodegradable polymer. The invention also provides a method by which various PHA synthases can be tested for their specificity with respect to different monomer substrates.

#The potential uses and applications of PHAs produced by PHA monomer synthases and PHA synthases include both medical and industrial applications. Medical applications of PHAs include surgical pins, sutures, staples, swabs, wound dressings, blood vessel replacements, bone replacements and plates, stimulation of bone growth by piezoelectric properties, and biodegradable carrier for long-term dosage of pharmaceuticals. Industrial applications of PHAs include disposable items such as baby diapers, packaging containers, bottles, wrappings, bags, and films, and biodegradable carriers for long-term dosage of herbicides, fungicides, insecticides, or fertilizers.

#In animals, the biosynthesis of fatty acids de novo from malonyl-CoA is catalyzed by FAS. For example, the rat FAS is a homodimer with a subunit structure consisting of 2505 amino acid residues having a molecular weight of 272,340 Da. Each subunit consists of seven catalytic activities in separate physical domains (Amy et al., Proc. Natl. Acad. Sci. USA, 86, 3114 (1989)). The physical location of six of the catalytic activities, ketoacyl synthase (KS), malonyl/acetyltransferase (M/AT), enoyl reductase (ER), ketoreductase (KR), acyl carrier protein (ACP), and thioesterase (TE), has been established by (1) the identification of the various active site residues within the overall amino acid sequence by isolation of catalytically active fragments from limited proteolytic digests of the whole FAS, (2) the identification of regions within the FAS that exhibit sequence similarity with various monofunctional proteins, (3) expression of DNA encoding an amino acid sequence with catalytic activity to produce recombinant proteins, and (4) the identification of DNA that does not encode catalytic activity, i.e., DNA encoding a linker region. (Smith et al., Proc. Natl. Acad. Sci. USA, 73, 1184 (1976); Tsukamoto et al., J. Biol. Chem., 263, 16225 (1988); Rangan et al., J. Biol. Chem., 266, 19180 (1991)).