Abstract
Chimeric plasmids, which were useful as cloning vehicles in a Streptococcus sanguis (Challis) host vector system, have been constructed. By using three different strategies of restriction endonuclease digestion and ligation, a deoxyribonucleic acid (DNA) fragment bearing an erythromycin resistance determinant was ligated in vitro to a phenotypially cryptic plasmid from Streptococcus ferus. Recombinant plasmids could be recovered after transformation of S. sanguis (Challis) with these preparations. Three useful chimeras were constructed. pVA680, 5.5 megadaltons in size, contained a single KpnI site into which passenger DNA may be spliced. pVA736, 5.0 megadaltons in size, contained single EcoRI, HindIII, and KpnI sites into which passenger DNA may be spliced. The EcoRI and KpnI sites of pVA736 may be used in combination with one another when ligating DNA into this plasmid. pVA738, 3.7 megadaltons in size, contained single HindIII and AvaI sites into which passenger DNA may be spliced. pVA680, pVA736, and pVA738 were stably maintained as multicopy plasmids in S. sanguis (Challis). None of them continued to replicate (amplify) in chloramphenicol-treated cells. By using pVA736 as a vector, we have cloned a chloramphenicol resistance determinant obtained from a large, conjugative streptococcal R plasmid. In addition, chromosomal DNA sequences from Streptococcus mutans have been inserted into pVA736 by using the KpnI-EcoRI site combination.