Positive selection vectors based on palindromic DNA sequences

Analysis of putative DNA amplification genes in the element AUD1 of Streptomyces lividans 66

The amplifiable AUD1 element of Streptomyces lividans 66 consists of two copies of a 4.7 kb sequence flanked by three copies of a 1 kb sequence. The DNA sequences of the three 1 kb repeats were determined. Two copies (left and middle repeats) were identical: (1009 bp in length) and the right repeat was 1012 bp long and differed at 63 positions. The repeats code for open reading frames (ORFs) with typical Streptomyces codon usage, which would encode proteins of about 36 kD molecular weight. The sequences of these ORFs suggest that they specify DNA-binding proteins and potential palindromic binding sites are found adjacent to the genes. The putative amplification protein encoded by the right repeat was expressed in Escherichia coli.

pKSS--a second-generation general purpose cloning vector for efficient positive selection of recombinant clones

A new small plasmid vector (pKSS) for the direct selection of insert-containing plasmid clones is presented. The selection strategy is based on the acquired sensitivity of Escherichia coli cells to p-chloro-phenylalanine (p-Cl-Phe) if they carry a pheS allele encoding a phenylalanyl-tRNA synthetase alpha subunit with relaxed substrate specificity. This pheS allele is present on pKSS. Insertion into, or replacement of, the plasmidial pheS gene by a cloned fragment enables transformed pheS wild-type cells to survive on agar plates containing p-Cl-Phe plus ampicillin. This host strain-independent positive selection of recombinant clones proved to be highly efficient (> 99%) and did not require purification of the vector fragment prior to cloning. The high-copy-number vector pKSS offers a multitude of restriction sites and all of the features for analysis of cloned fragments that stem from the cloning vector pBluescript (Stratagene, La Jolla, CA, USA). Thus, pKSS represents a valuable alternative to previously reported positive-selection vectors; it should prove particularly useful for cloning when expecting a high fraction of cells transformed with non-recombinant vector, and for construction of DNA libraries.