ABSTRACT
We introduce a method of in vitro recombination or "DNA shuffling" to generate libraries of evolved enzymes. The approach relies on the ordering, trimming, and joining of randomly cleaved parental DNA fragments annealed to a transient polynucleotide scaffold. We generated chimeric libraries averaging 14.0 crossovers per gene, a several-fold higher level of recombination than observed for other methods. We also observed an unprecedented four crossovers per gene in regions of 10 or fewer bases of sequence identity. These properties allow generation of chimeras unavailable by other methods. We detected no unshuffled parental clones or duplicated "sibling" chimeras, and relatively few inactive clones. We demonstrated the method by molecular breeding of a monooxygenase for increased rate and extent of biodesulfurization on complex substrates, as well as for 20-fold faster conversion of a nonnatural substrate. This method represents a conceptually distinct and improved alternative to sexual PCR for gene family shuffling.
Subject(s)
Genetic Techniques , Recombination, Genetic , Alleles , Amino Acid Sequence , Crossing Over, Genetic , DNA, Complementary/metabolism , Gene Library , Molecular Sequence Data , Mutagenesis , Mutation , Nocardia/genetics , Polymerase Chain Reaction , Polymorphism, Restriction Fragment Length , Rhodococcus/genetics , Sequence Homology, Amino Acid , Substrate SpecificityABSTRACT
Oxidation of C1-C4 primary alcohols in thermotolerant Bacillus methanolicus strains is catalyzed by an NAD-dependent methanol dehydrogenase (MDH), composed of ten identical 43,000-Mr subunits. Each MDH subunit contains a tightly, but non-covalently, bound NAD(H) molecule, in addition to 1 Zn2+ and 1-2 Mg2+ ions. The NAD(H) cofactor is oxidized and reduced by formaldehyde and methanol, respectively, while it remains bound to the enzyme. Incubation of MDH with methanol and exogenous NAD (coenzyme) results in reduction of this NAD coenzyme. Both NAD species are not exchanged during catalysis. NAD thus plays two different and important roles in the MDH-catalyzed reaction, with the bound NAD cofactor acting as primary electron acceptor and the NAD coenzyme being responsible for reoxidation of the reduced cofactor. MDH obeys a ping-pong type reaction mechanism, which is consistent with such a temporary parking of reducing equivalents at the MDH-bound cofactor. Spectral studies show that, in the presence of exogenous NAD and Mg2+ ions, MDH interacts with a previously identified 50,000-Mr activator protein. The activator protein appears to facilitate the oxidation of the reduced NADH cofactor of MDH, which results in a strongly increased turnover rate of MDH.
Subject(s)
Alcohol Oxidoreductases/chemistry , Alcohol Oxidoreductases/metabolism , Bacillus/enzymology , Kinetics , Molecular Structure , Molecular Weight , NAD/chemistry , Oxidation-Reduction , Protein Conformation , SpectrophotometryABSTRACT
The actinomycete Amycolatopsis methanolica was found to employ the normal bacterial set of glycolytic and pentose phosphate pathway enzymes, except for the presence of a PPi-dependent phosphofructokinase (PPi-PFK) and a 3-phosphoglycerate mutase that is stimulated by 2,3-bisphosphoglycerate. Screening of a number of actinomycetes revealed PPi-PFK activity only in members of the family Pseudonocardiaceae. The A. methanolica PPi-PFK and 3-phosphoglycerate mutase enzymes were purified to homogeneity. PPi-PFK appeared to be insensitive to the typical effectors of ATP-dependent PFK enzymes. Nevertheless, strong N-terminal amino acid sequence homology was found with ATP-PFK enzymes from other bacteria. The A. methanolica pyruvate kinase was purified over 250-fold and characterized as an allosteric enzyme, sensitive to inhibition by P(i) and ATP but stimulated by AMP. By using mutants, evidence was obtained for the presence of transketolase isoenzymes functioning in the pentose phosphate pathway and ribulose monophosphate cycle during growth on glucose and methanol, respectively.
