Fine mapping and DNA sequence of replication functions of Bacillus thuringiensis plasmid pTX14-3.

pTX14-3 is a 7.5-kb cryptic plasmid isolated from a Bacillus thuringiensis subspecies israelensis strain. Like many other small plasmids in gram-positive bacteria, pTX14-3 replicates via a single-stranded DNA intermediate. The nucleotide sequence of the replication region was determined and an open reading frame of 636 base pairs encoding a protein necessary for plasmid replication was identified by deletion analysis. No significant homology was found between this open reading frame and those encoding replication proteins identified on other plasmids isolated from gram-positive bacteria, nor could we find any homology to plus origins from other single-stranded DNA plasmids. Consequently, it seems that the replicon of pTX14-3 belongs to a new family of replicons in the group of single-stranded DNA plasmids. The sequence of the single-strand origin (i.e., the minus origin) responsible for the conversion of single-stranded plasmid DNA to double-stranded plasmid DNA was also determined. A partial homology between the minus origin of pTX14-3 and the Bacillus subtilis plasmid pBAA1 was identified. A previously identified locus that suppresses formation of high molecular weight multimers was also minimized and sequenced.