The structural stability of an expression plasmid bearing a heterologous cloned gene depends on whether this gene is expressed or not

Evaluation of the hok/sok killer locus for enhanced plasmid stability

The effectiveness of the hok/sok plasmid stability locus and mechanism of cloned-gene loss was evaluated in shake-flask cultures. Addition of the hok/sok locus dramitically increasedapparent plasmid segregational stability to the hok/sok(-) control. In terms of the number of generations before 10%of the population became plasmid-free, segregational stability was increased by 11- to 20-fold in different media in the absence of induction of the cloned-gene (hok/sok(+) plasmid stable for over 200 generations in all media tested). With constant expression of beta-galactosidase in the absence of an tibiotic, the segregational stability of the plasmid containing hok/sok was incresed more than 17- to 30-fold when beta-galactosidase was expressed at 7-15 wt % of total cell protein. Although the hok/sok system stabilized the plasmid well infour different media (Luria-Bertani (LB), LB glucose, M9C Trp, and a representative fedbatch medium), the ability of hok/sok to maintain the plasmid with induction of the cloned gene decreased as the complexity of the media increased. This result is better interpreted in terms of the influence of cloned-gene expression on plasmidmaintenance; plasmid segregational stability decreased linearly as specificbeta-galactosidase activity increased. (c) 1994 John Wiley & Sons, Inc.

Evolution of a recombinant (gucoamylase-producing) strain of Fusarium venenatum A3/5 in chemostat culture

Fusarium venenatum JeRS 325 is a transformant of strain A3/5 which produces Aspergillus niger glucoamylase (GAM) under the control of a Fusarium oxysporum trypsin-like protease promoter. The evolution of JeRS 325 was studied in glucose-limited chemostat cultures grown on NaNO3 or (NH4)2SO4 as the nitrogen source. Thirteen mutants which were more highly branched and four mutants which were more sparsely branched than the parental strain were isolated from the NaNO3 chemostat. The highly branched mutants detected in this chemostat did not displace the sparsely branched population. The mutants isolated from the NaNO3 chemostat complemented representative strains previously isolated from glucose-limited chemostat cultures of F. venenatum A3/5 grown on (NH4)2SO4, but showed little complementation between themselves. By contrast, a highly branched mutant isolated from the (NH4)2SO4 chemostat culture displaced the sparsely branched mycelial population. None of the mutants isolated from the NaNO3 or (NH4)2SO4 chemostats produced as much GAM as JeRS 325. Southern blot analysis showed that all except one mutant had lost copies of both the glucoamylase and the acetamidase (the selectable marker) genes. However, specific GAM production was not necessarily correlated with the extent of glaA gene loss observed. Further, 10 of the mutants had lost the ability to grow on acetamide as the sole nitrogen source, although they retained copies of the amdS gene. In competition studies, mutants which could not utilize acetamide displaced mutants which could. The presence of foreign DNA in JeRS 325 resulted in a reduced specific growth rate (compared to A3/5), but the presence of the foreign DNA did not prevent the evolution of the strain or the isolation of mutants which had improved growth rates.

Metabolic load and heterologous gene expression

The expression of a foreign protein(s) in a recombinant host cell or organism often utilizes a significant amount of the host cell's resources, removing those resources away from host cell metabolism and placing a metabolic load (metabolic drain, metabolic burden) on the host. As a consequence of the imposed metabolic load, the biochemistry and physiology of the host may be dramatically altered. The numerous physiological changes that may occur often lowers the amount of the target foreign protein that is produced and eventually recovered from the recombinant organism. In this review the physiological changes to host cells, the causes of the phenomenon of metabolic load, and several strategies to avoid some of the problems associated with metabolic load are elaborated and discussed.