Sexual rivalry between plasmids unlocked
doi:10.1038/nindia.2014.147 Published online 31 October 2014
An Indo-US team of researchers claim to have found how two rival plasmids of a bacteria resolve a "sexual conflict" — a study that may help find ways to stop the spread of antibiotic resistance from one bacteria to another.
In addition to chromosomal DNA, bacteria possess extra-chromosomal DNA elements called plasmids. They are frequently transferred between bacteria through a mechanism that plays the role of sex. Thus, successful transfer of a plasmid from donor to recipient bacteria is a measure of its fertility. However, it has been observed that on occasions when two plasmids co-reside in a single bacterial host, there is a rivalry between them. One of them prevents the sexual transmission of the other by means of a mysterious protein called “fertility inhibition factor (FIN)”.
Using a multi-pronged approach, a team of scientists from International Centre for Genetic Engineering and Biotechnology, New Delhi, University of Texas Health Science Centre at Houston and National Institute of Health, Bethesda, USA uncover the mechanism of a hitherto functionally opaque FIN called Osa. This is a protein expressed by the plasmid pSa, which prevents the transfer of T-DNA from the Ti (tumour inducing) plasmid of Agrobacterium to plant cells.
The researchers have found Osa to be an unusual enzyme with a single active site possessing both nuclease and ATPase activities. The authors show that Osa associates with the secretory apparatus (Type-IV secretion system-T4SS) through which DNA is transferred, and cleaves the T-DNA of the rival plasmid.
"The current finding paves the way for understanding the mechanisms of plasmid fertility inhibition," the authors report. Plasmids are frequently the carriers of multiple-antibiotic resistant traits and are the primary means by which antibiotic resistance spread between bacteria. Understanding the mechanisms of fertility inhibition might help us devise new strategies to counter horizontal transfer of such genes by plasmids and other conjugative elements, says the report.
1. Maindola, P. et al. Multiple enzymatic activities of ParB/Srx superfamily mediate sexual conflict among conjugative plasmids. Nat. Commun. 5:5322. doi: 10.1038/ncomms6322 (2014)