The partition functions of P1, P7, and F miniplasmids.

The partition regions of P1, P7, and F miniplasmids are discrete DNA sequences of about 3 kb in length that will promote accurate partition of hybrid plasmids independent of the source of replication functions or the position or orientation of the elements. Each of the par regions seems to be very similarly organized, with open reading frames for essential proteins and a terminal site which appears to be analogous to the centromere of eukaryotic cells. When cloned, these terminal sites exert incompatibility against their respective parent plasmids presumably because they can compete with the parent plasmids as substrates for partition. We have determined the complete DNA sequence of the P1 par region. In addition to the open reading frame for the essential parA protein (42-44 kd), the region contains a second open reading frame which could encode a 38-kd protein. The 2 large open reading frames appear to form an operon that is negatively regulated from a site adjacent to the promoter and responds to the par gene products in trans. Both this site and the downstream "centromere" site, incB, contain blocks of extremely AT-rich sequences, which are postulated to be binding sites for par proteins. The incB and upstream AT-rich regions both contain 20-bp imperfect inverted repeats. Further downstream from the minimal incB sequence (172 bp) lies an additional region which is essential for partition. The further analysis of the P1 par region should be greatly facilitated by the finding that it can function in cis to stabilize pBR322 vectors under conditions where the copy number of pBR322 is reduced.