The broad-host-range plasmid RK2 is capable of replication and stable maintenance within a wide range of gram-negative bacterial hosts. It encodes the essential replication initiation protein TrfA, which binds to the host initiation protein, DnaA, at the plasmid origin of replication (oriV). There are two versions of the TrfA protein, 44 and 33 kDa, resulting from alternate in-frame translational starts. We have shown that the smaller protein, TrfA-33, and its 64-residue amino-terminal peptide (designated T1) physically interact with the Escherichia coli beta sliding clamp (beta(2)). This interaction appears to be mediated through a QLSLF peptide motif located near the amino-terminal end of TrfA-33 and T1, which is identical to the previously described eubacterial clamp-binding consensus motif. T1 forms a stable complex with beta(2) and was found to inhibit plasmid RK2 replication in vitro. This specific interaction between T1 and beta(2) and the ability of T1 to block DNA replication have implications for the previously reported cell lethality caused by overproduction of T1. The toxicity of T1 was suppressed when wild-type T1 was replaced with mutant T1, carrying an LF deletion in the beta-binding motif. Previously, T1 toxicity has been shown to be suppressed by Hda, an intermediate regulatory protein which helps prevent over-initiation in E. coli through its interaction with the initiator protein, DnaA, and beta(2). Our results support a model in which T1 toxicity is caused by T1 binding to beta(2), especially when T1 is overexpressed, preventing beta(2) from interacting with host replication proteins such as Hda during the early events of chromosome replication.