Brevibacterium linens RBL strain cryptic plasmid pRBL1 (8.0 kb) is described. A region involved in pRBL1 autonomous replication in Corynebacterium glutamicum was identified by insertion and deletion mapping and partially sequenced. This region encodes for a hypothetical 310-amino acid (aa) protein closely related to the replicases of plasmids pXZ10142 (C. glutamicum) and pAL5000 (Mycobacterium fortuitum). The 310-aa protein also shows significant homology to proteins of pColE5-099 (Shigella sonnei) and pJD1 (Neisseria gonorrhoea). At least one of these proteins, the Rep protein of pColE5-099, is known to be involved in theta replication.