Tetrahymena ORC contains a ribosomal RNA fragment that participates in rDNA origin recognition

The Tetrahymena thermophila ribosomal DNA (rDNA) replicon contains dispersed cis-acting replication determinants, including reiterated type I elements that associate with sequence-specific, single-stranded binding factors, TIF1 through TIF4. Here, we show that TIF4, previously implicated in cell cycle-controlled DNA replication and rDNA gene amplification, is the T. thermophila origin recognition complex (TtORC). We further demonstrate that TtORC contains an integral RNA subunit that participates in rDNA origin recognition. Remarkably, this RNA, designated 26T, spans the terminal 282 nts of 26S ribosomal RNA. 26T RNA exhibits extensive complementarity to the type I element T-rich strand and binds the rDNA origin in vivo. Mutations that disrupt predicted interactions between 26T RNA and its complementary rDNA target change the in vitro binding specificity of ORC and diminish in vivo rDNA origin utilization. These findings reveal a role for ribosomal RNA in chromosome biology and define a new mechanism for targeting ORC to replication initiation sites.