Cloning vectors for Streptococcus thermophilus derived from a native plasmid.

A 3.5-kb native plasmid (pND103) was identified in Streptococcus thermophilus ST2-1. Preliminary sequence analysis indicated that pND103 belongs to group I S. thermophilus plasmids. A region of approximately 2 kb appears to contain three components: a plus origin of replication (ori) typical of plasmids that replicate via rolling circle replication; a gene encoding a replication protein (rep); and a gene encoding a small heat shock protein (hsp). pND103 was then used to construct S. thermophilus/Escherichia coli hybrid cloning vectors by ligating different portions of pND103 to an origin-probe vector (pND330) composed of pUC19 and a Gram-positive erythromycin resistance gene. The shuttle vectors (pND913, pND914 and pND915) were successfully introduced back into plasmid-free S. thermophilus ST3-1 as well as to Lactococcus lactis LM0230 and E. coli JM109. Segregational and structural stability study indicated that these vectors can be maintained in these hosts. The results indicated that pND913, pND914 and pND915 are potential shuttle cloning vectors for S. thermophilus.