Comparison of transformation protocols in Streptococcus gordonii and evaluation of native promoter strength using a multiple-copy plasmid

An active area of research in the development of Streptococcus gordonii for use as a bacterial commensal vector involves the identification and utilization of strong promoters for high-level expression of heterologous products. Escherichia coli plasmid vectors containing different streptococcal promoters often fail to become established in E. coli for unknown reasons. Therefore, it is desirable at times to transform S. gordonii, which is naturally competent, with small quantities of nascently ligated DNA without using E. coli first to amplify or screen the product. By comparing the efficiency of two methods used to induce competence in S. gordonii, it was shown that the use of a synthetic competence stimulating peptide substantially enhanced plasmid uptake by S. gordonii. We amplified the amylase-binding protein (abpA) promoter from the S. gordonii genome and, using a synthetic peptide to induce competence, directly introduced plasmid DNA containing this promoter into S. gordonii as an unamplified product of ligation. This plasmid facilitated abundant secretion of a heterologous product by S. gordonii. By assessing the levels of heterologous product secreted by two plasmid constructs, it was possible to evaluate the relative strength of two native promoters.