Figure 1820

Pheromones Induce Mating in Gram-positive Bacteria

In gram-positive bacteria such as Enterococcus, cells without plasmids secrete pheromones to attract bacteria with transferable (Tra+) plasmids. Mating pheromones bind to receptors on the surface of cells containing Tra+ plasmids. Binding the receptor activates the transfer genes to form a conjugation bridge and transfer the plasmid by rolling circle replication. Each pheromone is specific and only attracts bacteria with certain plasmids.

Mating pheromone

Receptor binds pheromone

Mating pheromone

Gram-positive cell without plasmid

Chromosome

Receptor binds pheromone

Chromosome

Tra+ plasmid

Gram-positive cell without plasmid

Gram-positive cell with transferable plasmid

Gram-positive cell with transferable plasmid

Transfer of plasmids between gram-positive bacteria is often promoted by pheromones.

there is considerable interest in using gram-positive bacteria as hosts in genetic engineering. Unfortunately, the genetics of gram-positive bacteria is far behind that of the intensively studied E. coli and its relatives. Nonetheless, mechanisms of gene-transfer are available in gram-positive bacteria.

Self-transmissible plasmids are widespread among gram-positive bacteria and many of these plasmids are rather promiscuous. Since the cell envelope is simpler in gram-positive bacteria, plasmid transfer is also simpler and a sex pilus is not needed. Apparently only half-a dozen genes are required to encode the transfer functions. Some gram-positive bacteria, such as Enterococcus, secrete mating pheromones into the culture medium. These are short peptides that induce the tra genes of plasmids in neighboring bacteria. This results in aggregation and plasmid transfer (Fig. 18.20). Different pheromones are specific for different plasmids. Only bacteria that lack a particular plasmid secrete the corresponding pheromone. Furthermore, the plasmid only expresses its transfer genes when a suitable recipient is nearby.

Gram-positive bacteria also harbor conjugative transposons (e.g. Tn916 of Ente-rococcus). These can transfer themselves from one bacterial cell to another (see Ch. 15). Apparently these elements excise themselves temporarily from the chromosome of the donor cell before conjugation. Once inside the recipient, they re-insert themselves into the bacterial chromosome.

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