Methods for increasing molecular diversity of polyketides and non-ribosomal proteins

A process for producing a library of two or more mutant modular polyketide synthase encoding strains of cells which express mutant functional polyketide synthases having an increased or reduced number of modules and which have been formed by recombination events which process comprises the steps of:

• (i) contacting a modular polyketide synthase encoding host strain of cells which is a homologous host for the polyketide synthase with a vector which includes a selectable marker, an inducible origin of replication and a portion of DNA homologous to a portion of DNA within the polyketide synthase, thereby to integrate the vector into cells of the strain within the polyketide synthase encoding gene cluster of the genome of said cells, such that the vector integrates into cells of the strain within the polyketide synthase encoding gene cluster via a single crossover event;
• (ii) applying selective pressure to the cells into which the vector has been integrated by inducing the origin of replication so that the cells eliminate the selectable marker through one or more recombination events leading to a mixture of strains of cells in which the cells of the strains express mutant functional polyketide synthases having an increased or reduced number of modules as compared with the strain of step (i);
• and
• (iii) screening for or selecting two or more strains of cells that lack the selectable marker and which express mutant functional polyketide synthases having an increased or reduced number of modules.