Cloning and expression of ?-galactosidase gene from Streptococcus thermophilus in Saccharomyces cerevisiae

Amplification of the Escherichia coli lacZ gene in Bacillus subtilis and its expression on a by-product growth medium

A Bacillus subtilis wild type strain and a kinA (spoIIJ) isogenic mutant were compared as hosts for the expression of the Escherichia coli beta-galactosidase gene, lacZ, driven by the B. subtilis aprE promoter in a chromosomal system. The 2 x SG sporulation formula, with some modifications, was used as a basal medium. The specific activity values recorded by the mutant strain at the stationary phase were markedly higher than those achieved by the wild type host. Exposure of the cells to increasing levels of chloramphenicol resulted in significant amplifications of the lacZ region. Gene copy numbers of 19 and 11 were estimated in the amplified wild type and kinA strains, respectively, with high segregational stability records. The magnitude of beta-galactosidase over-expression was dependent on, and roughly proportional to antibiotic resistance levels. Among five examined by-products, a 2.3-times diluted concentration of neutralized cheese whey was successfully used as a sole medium component for over-expression of the recombinant beta-galactosidase gene in B. subtilis.

Directed genomic integration, gene replacement, and integrative gene expression in Streptococcus thermophilus

Several pGEM5- and pUC19-derived plasmids containing a selectable erythromycin resistance marker were integrated into the chromosome of Streptococcus thermophilus at the loci of the lactose-metabolizing genes. Integration occurred via homologous recombination and resulted in cointegrates between plasmid and genome, flanked by the homologous DNA used for integration. Selective pressure on the plasmid-located erythromycin resistance gene resulted in multiple amplifications of the integrated plasmid. Release of this selective pressure, however, gave way to homologous resolution of the cointegrate structures. By integration and subsequent resolution, we were able to replace the chromosomal lacZ gene with a modified copy carrying an in vitro-generated deletion. In the same way, we integrated a promoterless chloramphenicol acetyltransferase (cat) gene between the chromosomal lacS and lacZ genes of the lactose operon. The inserted cat gene became a functional part of the operon and was expressed and regulated accordingly. Selective pressure on the essential lacS and lacZ genes under normal growth conditions in milk ensures the maintenance and expression of the integrated gene. As there are only minimal repeated DNA sequences (an NdeI site) flanking the inserted cat gene, it was stably maintained even in the absence of lactose, i.e., when grown on sucrose or glucose. The methodology represents a stable system in which to express and regulate foreign genes in S. thermophilus, which could qualify in the future for an application with food.