Excisable cassettes: new tools for functional analysis of Streptomyces genomes.

The functional analysis of microbial genomes often requires gene inactivation. We constructed a set of cassettes consisting of single antibiotic resistance genes flanked by the attL and attR sites resulting from site-specific integration of the Streptomyces pSAM2 element. These cassettes can easily be used to inactivate genes by in-frame deletion in Streptomyces by a three-step strategy. In the first step, in Escherichia coli, the cassette is inserted into a cloned copy of the gene to be inactivated. In the second step, the gene is replaced by homologous recombination in Streptomyces, allowing substitution of the wild-type target gene with its inactivated counterpart. In the third step, the cassette can be removed by expression of the pSAM2 genes xis and int. The resulting strains are marker-free and contain an "attB-like" sequence of 33, 34, or 35 bp with no stop codon if the cassette is correctly chosen. Thus, a gene can be disrupted by creating an in-frame deletion, avoiding polar effects if downstream genes are cotranscribed with the target gene. A set of cassettes was constructed to contain a hygromycin or gentamicin resistance gene flanked by the attL and attR sites. The initial constructions carrying convenient cloning sites allow the insertion of any other marker gene. We tested insertion and excision by inserting a cassette into orf3, the third gene of an operon involved in spiramycin biosynthesis. We verified that the cassette exerted a polar effect on the transcription of downstream genes but that, after excision, complementation with orf3 alone restored spiramycin production.