|Antisense-RNA-mediated plasmid copy number control in pCG1-family plasmids,
pCGR2 and pCG1, in Corynebacterium glutamicum.
Microbiology 156: 3609-3623. 2010.
N. Okibe, N. Suzuki, M. Inui and H. Yukawa.
pCGR2 and pCG1 belong to different subfamilies of the pCG1 family of Corynebacterium glutamicum plasmids. Nonetheless, they harbor homologous putative antisense RNA genes,
crrI and cgrI, respectively. The genes in turn share identical positions complementary
to the leader region of their respective repA (encoding plasmid replication initiator) genes. Determination of their
precise transcriptional start- and end-points revealed the presence of
short antisense RNA molecules (72 bp, CrrI: and 73 bp, CgrI). These short
RNAs and their target mRNAs were predicted to form highly structured molecules
comprising stem-loops with known U-turn motifs. Abolishing synthesis of
CrrI and CgrI by promoter mutagenesis resulted in about sevenfold increase
in plasmid copy number on top of an 11-fold (CrrI) and 32-fold (CgrI) increase
in repA mRNA, suggesting that CrrI and CgrI negatively control plasmid replication.
This control is accentuated by parB, a gene that encodes a small centromere-binding plasmid-partitioning protein, and is located upstream of repA. Simultaneous deactivation of CrrI and parB led to a drastic 87-fold increase in copy number of a pCGR2-derived shuttle vector. Moreover, the fact that changes in the structure of the terminal loops of CrrI and CgrI affected plasmid copy number buttressed the important role of the loop structure in formation of the initial interaction complexes between antisense RNAs and their target mRNAs. Similar antisense RNA control systems are likely to exist not only in the two C. glutamicum pCG1 subfamilies but also in related plasmids across Corynebacterium species.