New vectors derives from pUC 18 for clonig and thermal-induced expression in Escherichia coli.

We report the construction of two vectors for Escherichia coli: pUC72, for molecular cloning, and pPLT7, for thermal-induced expression. The main feature of pUC72 is a novel polylinker region that includes restriction sites for Nde I and Nco I which provide an ATG codon for proper translation initiation of expressed genes. Vector pPLT7 is ideal for thermo-inducible expression in host cells that carry the cI857 repressor gene. The use of pPLT7 was validated by the successful expression of the genes encoding carp and porcine growth hormones. These vectors provide novel cloning possibilities in addition to simple, non-expensive, high level expression of recombinant proteins in E. coli.

Vectors pUC18 and pUC19 are small high-copy number plasmids that are widely used for cloning and manipulation of DNA fragments (9). One important feature of these plasmids is the presence of a multiple cloning site (MCS) within the coding region of the lacZα fragment which allows the detection of cells harboring recombinant plasmids by their inability to cleave the chromogenic substrate X-Gal (7). In order to facilitate the subcloning and transfer of cloned DNA sequences from pUC plasmids to other vectors we have modified the original MCS of pUC18 by introducing new restriction sites. First, the Nde I restriction site present in pUC18 was deleted after digestion of this plasmid with this enzyme following treatment with the Klenow enzyme and ligation with T4 DNA ligase. The resulting plasmid, named pUC28, was digested with Xba I and Hind III and then ligated to annealed synthetic oligonucleotides containing new restriction sites. The resulting vector, pUC72, contains the following novel restriction sites: Bgl II, Nde I and Nco I (Figure 1.A). The reading frame of the lacZα fragment was retained allowing the screening of recombinant plasmids by the white/blue selection on plates containing X Gal. In pUC72 we introduced the Nco I (5’-CCATGG-3’) and Nde I (5’-CATATG-3’) restriction sites because they contain a methionine codon (ATG) which can be used for proper translation initiation. This is a desirable feature not commonly found in pUC-derived vectors which allows inserts containing these sites at the 5´-ends to be readily cloned into this vector without the addition of extra amino acids at the N-terminus of the expressed protein. Furthermore, genes cloned into pUC72 are less likely to be translated because the cloning orientation of the insert does not allow in-frame fusion with the lacZα gene. This is particular relevant when the cloned gene codes for toxic or environmentally dangerous proteins.

Figure 1

General features of pUC72 (A) and pPLT7 (B). Only relevant restrictions sites are shown. MCS = multiple cloning site; SD = Shine Dalgarno region.

Most commercially available expression vectors require the synthetic IPTG inducer to promote transcription from the pLac promoter. These vectors are generally derived from pUC plasmids because of their high-copy number - a requirement for high level expression of recombinant proteins in E. coli. In this work we sought the construction of a bacterial expression vector based on the strong lambda phage pL promoter (3). First, a ~300 pb fragment containing the lambda phage pL promoter and phage T7 gene 10 Shine Dalgarno was amplified by PCR from pPLT4 (1) using primers O123 (5'-GAATTCCATATGTATATCTCCTTCTTAAAG-3') and O122 5'-GCGGAATTCCTACCAAACAATGCCCCCT-3'). The amplicon was digested with EcoR I and cloned into EcoR I-digested pUC28 resulting in vector pPLT7 (Figure 1.B).

The lambda phage pL promoter is negatively controlled by a repressor coded by the cI gene. A mutant form of cI (cI857) renders the repressor inactivate at temperatures above 37 °C. This behavior has been exploited in the establishment of an approach for controlled expression in E. coli based on growth temperature (2, 6, 8). The use of cI857 for regulated expression from the pL promoter is of great interest for industrial purposes since it offers an inexpensive and easy way to control the expression of recombinant proteins in E. coli. Recommended E. coli hosts for pPLT7 include N4830-1 and M5219 which contain the cI857 gene integrated in the genome (2, 4). The use of pPLT7 was tested in E. coli for the expression of two commercially important proteins: carp and porcine growth hormones. PCR-amplified fragments for both cDNA´s were cloned as Nde I-BamH I fragments into pUC72 and then transferred to pPLT7 digested with the same enzymes (data not shown). The resulting plasmids were used to transform N4830-1 or M5219 E. coli cells. Host cells transformed with these plasmids were incubated at 30 °C for 4 hours before shifting to the induction temperature (40-42°C). Samples (1 mL) were collected at different time points and after centrifugation pellets were washed with saline 0.9% and ressuspended in lysis buffer 2X (5). Figure 2 shows the protein expression profiles of two different E. coli strains transformed with pPLT7-derived constructs. In both cases, high level expression of recombinant carp and porcine growth hormones was achieved after a thermal shift and the heterologous proteins had the expected molecular mass (~22 kDa).

Figure 2

Protein expression profiles of E. coli cells harboring pPLT7 with cDNA sequences for carp (cGH) and porcine (pGH) growth hormones. (A) SDS-PAGE 17.5%. Lane 1, MW marker; lane 2, E. coli N4830-1 cells harboring cGH before thermal induction; lanes 3-5, samples collected 1, 2 and 3 hours after thermal induction at 42 °C, respectively. (B) SDS PAGE 12.5 %. Lane 1: E. coli M5219 cells harboring pGH before thermal induction; lanes 2 7, samples collected after intervals of 30 minutes until 3 hours after thermal induction at 40 °C; lanes 8 and 9, E. coli M5219 transformed with pPLT7 before and after thermal induction (40 °C), respectively; lane 10, MW marker. The arrows indicate the position of induced proteins (~22 kDa).

pUC-derived vectors have a spread use in molecular biology because they exhibit high copy number, genetic stability and multiple cloning sites for DNA manipulations. The plasmids described in this work provide novel additional features which may reveal useful for the manipulation of cloned DNA fragments and expression of recombinant proteins in E. coli.