Chimeric plasmids were constructed in order to establish if the tetracycline (Tc)-resistance (Tcr)-determinant that was contained on the Streptococcus faecalis plasmid pAM 1 could be expressed in Streptococcus sanguis. pAMAlpha1 does not transform these cells. The chimeras were constructed from a 5 kb EcoRI fragment that was derived from pAMBeta1 and a 3.3 kb EcoRI fragment that was derived from pAMAlpha1. The 5 kb fragment contained a replication origin that was known to function in S. sanguis whereas the 3.3 kb fragment contained a Tcr-determinant and a replication origin that was known to function in Bacillus subtilis. When these chimeras were added to a competent culture of S. sanguis Wicky, a family of Tcr plasmids, of various sizes, was isolated from them. The replication of members in this family of plasmids, designated pRAN1, pRAN4, pRAN5, pRAN16, and pRAN20, is being investigated. These plasmids are of interest because they contain totally or in part two, independent origins of replication. One of these origins, the one derived from pAMAlpha1, is known to be present in other naturally occurring plasmids that are found in a variety of Gram-positive bacteria. Work in progress is designed to determine which of the two origins of replication is the functional one for each member of the family. In addition, experiments are underway that will provide an understanding as to why pAMAlpha1 does not replicate in S. sanguis.
