Demonstration of the essentiality of dxr. (A) The chromosomal location of the dxr operon, the region deleted in the delivery vector (pORTAL3), and the regions present in the complementing vectors pOPW (complete operon) and pOPD (operon without dxr) are shown. (B) The deletion delivery vector pORTAL3 was introduced into the chromosome by homologous recombination followed by site-specific recombination with the complementing vector pOPW to generate a merodiploid strain. DCOs were generated in this background and analysed. Southern analysis of representative DCOs isolated in the merodiploid background is shown. Genomic DNA was digested with NotI and hybridised to the indicated probe. The sizes for the wild-type and deletion alleles are shown diagrammatically – replacement of the wild-type gene results in an additional NotI site in the chromosome. Lane 1 – wild-type DCO (dxrwt, dxrint), Lane 2 – Deletion DCO (dxrΔ, dxrint), Lane 3 – markers. The bands for the wild-type, deletion and integrated copies of dxr are indicated.