Plasmid-Encoded Ribonucleotide Reductases: Unveiling the Hidden Diversity

Ribonucleotide reductase (RNR) catalyzes the synthesis of deoxyribonucleotides by reducing the corresponding ribonucleotides. Plasmid pKOS1-1-3 from multidrug-resistant Citrobacter portucalensis strain KOS1-1 contained the nrdAB genes for class Ia RNR. The C. portucalensis chromosome encodes a phylogenetically distant class 1a RNR. Plasmid pKOS1-1-3 contained both genes responsible for its replication and maintenance, as well as the genes for bacteriophage structural proteins, suggesting that it could be a phage-plasmid hybrid. Probably, pKOS1-1-3 uses its RNRs to increase the nucleotides pool, allowing for more active replication during lytic infection. Meta-analysis revealed 5568 RNR genes on 2903 plasmids in various bacterial lineages, and 1082 of these were predicted to be phage-plasmids. We propose that plasmid RNR genes are of phage origin and could be functionally important for phage-plasmids.