Cloning of penicillin G acylase-encoding gene from Escherichia coli high-producing strain RE3

IS2-mediated re-arrangement of the promoter sequence suppresses metabolic burden of the recombinant plasmid

Recombinant plasmid pKA18 of the high expression bacterial system for penicillin amidase ('penicillin G acylase') bears the 3' end region of IS2 element. The IS2 sequence replaces the -35 region of promoter of pga and extends up to TAGTAT box at position -10 of the promoter region. It therefore forms a hybrid promoter of pga ppgaHT. A natural promoter ppgaWT was not detected on any recombinant plasmid isolated from recombinant strains of Escherichia coli constitutively producing penicillin amidase. PCR fragments carrying both types of promoters were cloned into the promoter-probe vector pET2 to compare their transcriptional activity: the activity of ppgaWT was 5x higher than that of ppgaHT. The same nucleotide "G" localized 28 nucleotides upstream of the translation start point was identified as the respective transcription start point of both mRNAs. An attempt was made to place the pga gene cloned on a plasmid under the control of the natural promoter: not a single clone expressing penicillin amidase was found among 150 transformants. High transcriptional activity of the natural promoter together with high pga gene dosage could result in a deleterious metabolic burden of the periplasmic enzyme.

Cryptic plasmid pRK2 from Escherichia coli W: sequence analysis and segregational stability

Cryptic plasmid pRK2 of the strain Escherichia coli W (ATCC 9637), an ancestor of production strains for penicillin G acylase, was sequenced and characterized. Based on the data on replication region and origin (ori sequence AAC, 924-926nt), the plasmid was classified as ColE1-like plasmid. DNA sequence analysis revealed five orfs hypothetical products of which shared a significant sequence similarity with putative proteins encoded by DNA of plasmid pColE1. orf1 codes for protein Rom involved in the control of plasmid replication, orfs 2-5 code for putative mobilization proteins (Mob A-D) that show a high level of similarity with the ones encoded by DNA of plasmids pColE1 and pLG13 (E. coli), pECL18 and pEC01 (Enterobacter cloacae), pSFD10 (Salmonella choleraesuis), and pScol7 (Shigella sonnei). Recombinant plasmids pRS11 (4.91kbp), pRS12 (4.91kbp), pRS2 (2.996kbp), and pRS3 (2.623kbp) that bear the Spectinomycin resistance determinant (Spc(R)) were prepared on the basis of nucleotide sequence of pRK2. These constructs are stably maintained in the population of E. coli cells grown in the absence of the selection pressure for 63 generations. The copy number of Spc(R) constructs in E. coli host grown in antibiotic-free LB medium ranges from 25 to 40 molecules per chromosomal equivalent.

Recent biotechnological interventions for developing improved penicillin G acylases

Penicillin G acylase (PAC; EC 3.5.1.11) is the key enzyme used in the industrial production of beta-lactam antibiotics. This enzyme hydrolyzes the side chain of penicillin G and related beta-lactam antibiotics releasing 6-amino penicillanic acid (6-APA), which is the building block in the manufacture of semisynthetic penicillins. PAC from Escherichia coli strain ATCC 11105, Bacillus megaterium strain ATCC 14945 and mutants of these two strains is currently used in industry. Genes encoding for PAC from various bacterial sources have been cloned and overexpressed with significant improvements in transcription, translation and post-translational processing. Recent developments in enzyme engineering have shown that PAC can be modified to gain conformational stability and desired functionality. This review provides an overview of recent advances in the production, stabilization and application of PAC, highlighting the recent biotechnological approaches for the improved catalysis of PAC.

Evaluation of strains derived from Escherichia coli W as hosts for the expression of penicillin G-acylase-encoding gene cloned on the recombinant plasmid pKA18

The potential for production of penicillin G-acylase (PGA), encoded by the chromosomal gene pgai, of four strains belonging to a genealogical line derived from the strain Escherichia coli W, was evaluated in a medium with and without the inducer phenylacetic acid (PA). These strains were used as hosts of the recombinant plasmid pKA18, in which the structural gene pgac isolated from the strain RE3, the best host strain of a line giving the highest production, was cloned. The presence of the inducer reduced the copy number of the plasmid in all recombinant strains. Only in recombinant strain RE3 (pKA18) the reduction of the gene dosage of pgac resulted also in the reduction of the amount of PGA synthesized by the cells. The reduced activity of the cells did not result from a segregation of plasmid-free clones. Also the growth rate was decreased by 20 and 40% in the host and recombinant strains, respectively. The host strain RE3 showing the highest production of PGA was also the best host of the recombinant plasmid in terms of the segregational stability and copy number (198 copies per chromosome). The recombinant strain RE3 (pKA18) also provided the highest production of PGA.

A chemostat culture as a tool for the improvement of a recombinant E. coli strain over-producing penicillin G acylase

The recombinant strain RE3(pKA18) of Escherichia coli constitutively overproduces penicillin G acylase (PGA) from plasmid-borne gene pga. The host strain RE3 bears the same pga gene on its chromosome, the expression of which is controlled by the natural mechanism of induction with phenylacetic acid (PA). To evaluate the maximum biosynthetic capacity for PGA, induction of the chromosomal pga by PA was studied in a culture of the recombinant strain. PGA production by batch cultures of RE3(pKA18) and RE3 showed a different response to the addition of PA to the medium: while an addition of PA induces PGA in a culture of strain RE3 as expected, in recombinant cells it lowers the specific activity of PGA and a large amount of PGA is released into the culture medium. To improve the PGA production, the strain RE3(pKA18) was cultured in a carbon-limited chemostat and subjected to selection pressure in a medium supplemented with phenylacetic acid amide (PAA). Phenylacetic acid amide served as a source of nitrogen, an inducer of PGA and a factor exerting positive selection pressure on the maintenance of the recombinant plasmid. After 130 generations of growth in the presence of the inducer, no recombinant strain with constitutive expression of the chromosomal gene pga was detected in the prevailing P(+) subpopulation in the chemostat. Shake-flask experiments with the parent recombinant strain RE3(pKA18), host strain RE3, chemostat evolvant ERE3(epKA18), the cured host ERE3 alone, and its derivative after retransformation with ancestral plasmid ERE3(pKA18) showed that inactivation of the plasmid-borne pga by a frame-shift mutation (plasmid epKA18) occurred in the plasmid-bearing subpopulation accumulated in the chemostat. Marked adaptive changes evolved in the host ERE3 during a 130 generation culture: (1) the specific growth rate of the host increased by 30% in a medium without PA, (2) the copy number of plasmids pKA18 and epKA18 in the host cultured in PA-free medium dropped by about 40%, and (3) the leakage of PGA from the cell in the presence of PA found in strain RE3(pKA18) was not observed in strain ERE3(pKA18). This new recombinant strain with modified traits was constructed by means of retransformation of the evolved host ERE3 with ancestral plasmid pKA18.

DegP-coexpression minimizes inclusion-body formation upon overproduction of recombinant penicillin acylase in Escherichia coli

We demonstrated the enhancement of recombinant penicillin acylase (PAC) production in Escherichia coli by increasing the intracellular concentration of the periplasmic protease DegP. Using appropriate host/vector systems (e.g., HB101 harboring pTrcKnPAC2902 or MDDeltaP7 harboring pTrcKnPAC2902) in which the expression of the pac gene was regulated by the strong trc promoter, the overproduction of PAC was often limited by periplasmic processing and inclusion bodies composed of protein aggregates of PAC precursors were formed in the periplasm. The amount of these periplasmic inclusion bodies was significantly reduced and PAC activity was significantly increased upon coexpression of DegP. The specific PAC activity reached an extremely high level of 674 U/L/OD(600) for MDDeltaP7 harboring pTrcKnPAC2902 and pKS12 under optimum culture conditions. However, such improvement in the production of PAC was not observed for the expression systems (e.g., MDDeltaP7 harboring pCLL2902) in which the periplasmic processing was not the step limiting the production of PAC. The results suggest that DegP could in vivo assist the periplasmic processing though the enzyme is shown to be not absolutely required for the formation of active PAC in E. coli. In addition, the steps limiting the production of PAC are identified and the reasons for the formation of PAC inclusion bodies are discussed here.

Indigenous plasmids in a production line of strains for penicillin G acylase derived from Escherichia coli W

Three indigenous plasmids designated pRK1, pRK2 and pRK3 were identified among producers of penicillin G acylase (PGA) derived from the strain Escherichia coli W ATCC 9637. Their size and copy number (CN) in E. coli W were determined (kb; CN): pRK1 (80; 3.4), pRK2 (5.1; 71), and pRK3 (4.8; 13.7). Strain E. coli RE2 harboring these plasmids was used for selection of strains with reduced number of plasmids: the strain RE3 without plasmid pRK1 and the plasmid-less strain cERE3 were isolated. Indigenous plasmids did not code for the resistance determinants against 23 antibiotics and 10 heavy metals.