- The host-range specificity of a plasmid refers to the spectrum of bacterial species in which the plasmid can successfully replicate and be maintained.
- One of the primary factors influencing this specificity is the origin of replication (ori). The ori must be recognized by the host cell’s replication machinery. Some plasmids have broad-host-range origins that function across diverse bacterial genera, while others, such as the F plasmid in E. coli, have a narrow host range and replicate only in closely related species.
- Another important factor is the presence of plasmid-encoded replication initiator proteins (Rep proteins). These proteins initiate plasmid replication, and their functionality depends on compatibility with the host cell’s environment. If the host does not support the activity of these proteins—due to differences in accessory factors or DNA topology—the plasmid may fail to replicate.
- For a plasmid to be stably inherited over generations, it must also include functional partitioning and maintenance systems, such as those encoded by par genes. These systems ensure that plasmid copies are correctly distributed to daughter cells during cell division. In hosts where these systems do not function efficiently, plasmids may be gradually lost, even if they can replicate.
- The ability of a plasmid to express its genes in a new host also depends on promoter compatibility. Plasmid-encoded promoters must be recognized by the host’s RNA polymerase to ensure the transcription of essential genes, including those involved in replication, stability, and antibiotic resistance. Incompatible promoters may lead to poor expression and plasmid instability.
- Additionally, many bacteria possess restriction-modification systems that identify and degrade foreign DNA. If a plasmid lacks proper protective modifications, it may be targeted and destroyed upon entry into a new host cell. This limits the ability of the plasmid to establish itself in such bacteria.
- Finally, in the case of plasmids that spread via conjugation, successful transfer into a new host depends on the compatibility of the conjugative machinery and the presence of suitable surface receptors on the recipient cell. Even if the plasmid is capable of replication in the new host, it must first be able to enter it through an efficient conjugation process.
- In summary, a plasmid’s host range is shaped by how well its replication, gene expression, and maintenance systems are suited to the cellular environment and defense mechanisms of the potential host.
