[Construction and verification of the effectiveness of pMBL: a cloning vector of exported proteins encoding genes]

The beta-lactamase was used as the reporter of expression and transmembrane secretion in this paper. A fragment of Amp resistance gene encoding the mature part of beta-lactamase (delta P delta SP Amp, i.e. without promoter and signal peptide coding sequences) was amplified from pUC18 vector. The upstream primer has BglII, BclI, BamHI in three reading frame respectively, in order to in frame fuse and express target genes together with the downstream reporter in finally constructed vector. Meanwhile, pET-28 was digested with the restriction enzymes BglII and Bst1107 I. The 2.8 kb fragment with replication origin, Kan resistance gene and MCS was recovered, filled, self-ligated and resulted in a plasmid pKan-B. The Bgl II site on pKan-B was then filled and the plasmid pKan was obtained. The delta P delta SP Amp gene, which was first cloned into pGEM-T-EASY vector, was inserted into pKan between EcoR I and XbaI sites. A plasmid pMBL-E was selected, with which the bacteria host could grew on Kan plate but not on plate with both Amp and Kan. An EcoRI site beside HindIII on the plasmid pMBL-E was then filled, and the plasmid pMBL, a cloning vector of the exported proteins encoding genes was finally obtained. Both results of the restriction enzyme digestion and sequencing demonstrated the correctness of the construction. The Tet resistance gene, a transmemebrane protein encoding gene, was applied to verify the effectiveness of the reporter in the vector. Cut with EcoRI and BamHI, a 375 bp fragment including promoter and 96 animo acids coding sequence (including signal peptide) of Tet was obtained from pBR322 vector. The fragment was then ligated to the vector pMBL which had been cut with both enzymes of EcoRI and BglI, or EcoRI and BclI, or EcoRI and BamHI (as 0, +1, +2 respectively of the beta-lactamase gene reading frame). Kan and Amp double resistant colonies only grew with the EcoRI and BglII combination (0 position). Restriction enzyme digestion and sequencing results of the recombinant plasmid showed that Tet resistance gene, which promoted the expression and transmembrane secretion of downstream beta-lactamase, was inserted in a correct reading frame into the vector. Thus, the results verified the effectiveness of the constructed vector pMBL, which may be used effectively to clone genes encoding exported proteins with promoters and signal peptide sequences.