A replication terminus located at or near a replication checkpoint of Bacillus subtilis functions independently of stringent control

We have examined a replication terminus (omega L1) located on the left arm of the chromosome of Bacillus subtilis and within the yxcC gene and at or n ear the left replication checkpoint that is activated under stringent condi tions. The psi L1 sequence appears to bind to two dimers of the replication terminator protein (RTP) rather weakly and seems to possess overlapping co re and auxiliary sites that have some sequence similarities with normal Ter sites. Surprisingly, the asymmetrical, isolated psi L1 site arrested repli cation forks in vivo in both orientations and independent of stringent cont rol. In vitro, the sequence arrested DnaB helicase in both orientations, al beit more weakly than the normal Ter1 terminus, The key points of mechanist ic interest that emerge from the present work are: (i) strong binding of a Ter (psi L1) sequence to RTP did not appear to be essential for fork arrest and (ii) polarity of fork arrest could not be correlated in this case with just symmetrical protein-DNA interaction at the core and auxiliary sites o f psi L1. On the basis of the result it would appear that the weak RTP-L1Te r interaction cannot by itself account for fork arrest, thus suggesting a r ole for DnaB-RTP interaction.