Recombinant protein production in Antarctic Gram-negative bacteria.
This review reports some results from our laboratory on the setting up of a psychrophilic expression system for the homologous/heterologous protein production in cold-adapted bacteria by using natural plasmids as cloning vectors. By screening some Antarctic bacteria for the presence of extrachromosomal elements, we identified three new plasmids, pMtBL from Pseudoalteromonas haloplanktis TAC125, and pTAUp and pTADw, from Psychrobacter sp. TA144. The latter autoreplicating elements were isolated, cloned, and fully sequenced and their molecular characterisation was carried out; however, we focused our attention on the small multicopy plasmid, pMtBL, from the Gram-negative P. haloplanktis TAC125 strain. This episome turned out to be an interesting extrachromosomal element, since it displays unique molecular features as its transcriptional inactivity. Being cryptic, the inheritance of pMtBL totally relied on the efficiency of its replication function. This function was bound to a region of about 850 bp, identified by an in vivo assay based on the possibility to efficiently mobilize plasmidic DNA from a mesophilic donor (Escherichia coli) to psychrophilic recipient by intergeneric conjugation. This information was instrumental in the construction of a shuttle vector, able to replicate either in E. coli or in several cold-adapted hosts (clone Q). Since the conversion of a cloning system into an expression vector requires the insertion of transcription and translation regulative sequences, the corresponding signals from the aspartate aminotransferase gene isolated from P. haloplanktis TAC125 were inserted, generating the pFF vector. To investigate the possibility of obtaining recombinant proteins in this cold-adapted host, we used the psychrophilic alpha-amylase from the Antarctic bacterium P. haloplanktis TAB23 (previously known as Alteromonas haloplanktis A23) as a model enzyme to be produced. Our results demonstrate that the cold-adapted enzyme was not only produced but also efficiently secreted by the recombinant PhTAC125 cells. The described expression system represents the first example of heterologous protein production based on a true cold-adapted replicon.