Question

Compare and contrast properties of conjugation in Gram-negative bacteria and Firmicutes

Answer 1

Answers:

In Gram-negative bacteria, conjugation can be divided into two stages. The first stage involves the formation of a specific cytoplasmic bridge between the mating cells, whereby the plasmid-containing donor and the recipient make contact through extracellular conjugative pili, which have been broadly divided into two morphological groups: (1) long flexible pili and (2) short rigid pili . After the tip of the F-pilus (encoded by an extrachromosomal fertility (F) factor) initiates contact with the recipient cell, a cellular mating aggregate is formed and stabilized between the two cells. A DNA-transport pore extends through the cell envelopes of the joined donor and recipient and connects the cytoplasm of the pairing cells. In the second stage, which involves the transfer and processing of DNA, one strand of the plasmid DNA in the donor is nicked at the specific site of the origin of transfer (oriT), and a mobilization protein, Mob, is covalently attached to one end (5′ terminus) of the nicked DNA strand and interacts with the proteins of the transport pore to anchor this end of the transferring DNA strand to the pore. At the oriT site, DNA replication by the rolling-circle mode produces a single-stranded copy of the plasmid DNA that moves through the pore to the recipient cell. When the end (3′ terminus) of the transferring DNA strand reaches the pore, it joins covalently with the leading end (5′ terminus) of the same DNA strand, which is anchored there by the Mob protein-pore complex. As a result, a covalently closed single-stranded copy of the plasmid is introduced into the cytoplasm of the recipient, and its complementary strand is generated by DNA replication in the recipient. The double-stranded plasmid can then either replicate further or integrate into the recipient chromosome. When a plasmid becomes integrated into the donor chromosome, it can also transfer chromosomal genes to a recipient cell.

The Firmicutes are a phylum of bacteria, most of which have gram-positive cell wall structure. In firmicutes, the mechanism of conjugation is very different, as pili are not found, and no specific bridge between mating pairs of bacteria via a conjugative pilus is formed. At least two categories of conjugation that are mediated by self-transferable plasmids have been found. The third category involves conjugative transposons, One of the plasmid-conjugation systems in Gram-positive bacteria is represented by the pheromone-responding plasmids. These plasmids are confined to the enterococci and their close relatives. The size of the plasmids tends to be larger than 25 kb. During the transfer process, the recipient cells excrete sex pheromones composed of small peptides, which trigger the plasmid-carrying donor cell to produce surface proteins that promote cell clumping. This increases cell-to-cell contact between donor and recipient cells. The necessary cellular and molecular events for the conjugal transfer of plasmids are then induced. These pheromone-responding plasmids often contain genes encoding hemolysins, bacteriocins, or resistance to antibiotics

The second system of plasmid transfer occurs at variable frequencies (generally in the range of 10− 6 to 10− 3 per recipient), depending on the plasmid type and the genotype of the mating pair, and requires that donor and recipient cells be co-cultivated on a solid surface (i.e., direct contact between donors and recipients). These plasmids do not show a response to pheromone signals, and the mechanisms of their transfer are unclear.

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