Normal terC-region of the Bacillus subtilis chromosome acts in a polar manner to arrest the clockwise replication fork.

A procedure is described for relocating a functional terC-region to various sites on the Bacillus subtilis chromosome, and in alternative orientations. The relocated terC-region comprised the IRR-rtp portion of the chromosome contained within a 1.75 x 10(3) base-pair segment of DNA. This segment was first cloned into the Tn 917 vector pTV20 in both orientations, and the two new plasmids used for inserting the terC-region into chromosomal copies of Tn 917. When relocated to the pyr and metD loci (139 degrees and 100 degrees positions on the 360 degrees map) it was found that clockwise replication fork arrest occurred only when the IRR-rtp (or terC-) region was oriented, in relation to the direction of approach of the fork, in the same way as in the wild-type strain. Thus, the complete IRR when located in the chromosome, and apparently made up of opposing terminators which might enable it to function in both orientations, is polar in its action. Of the two inverted repeats present in the IRR, it appears that IRI is functional in the chromosome, but not IRII.