Research in the focus

Dr. Jörn Petersen and his team of scientists at the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures have jointly with Dr. John Vollmers and Dr. Anne-Kristin Kaster of the Karlsruhe Institute of Technology (KIT) identified a unique plasmid called pLA6_12, which occurs with a sequence identity of 100 percent in various species of Roseobacter bacteria. Initial investigations showed that the plasmid can increase the tolerance of marine bacteria to pollution – in this case, highly toxic and carcinogenic chromate (CrO42-) – by a factor of 20, thereby conveying an important evolutionary advantage. The researchers recently published their findings in the renowned journal ‘Proceedings of the National Academy of Sciences’ (PNAS).

Horizontal gene transfer by plasmids 
Plasmids are circular, double-stranded DNA molecules that occur in bacterial cells independently from the bacterial chromosome, and are used by bacteria for horizontal gene transfer. DNA information is transmitted from one bacterial cell to another with the help of an injection mechanism. Inter alia, this exchange of genetic information plays an important role in antibiotic resistance. While there is detailed information available on the spread of antibiotic resistance, only little is known about naturally occurring plasmid-transfers. “The plasmid’s 100 percent sequence identity that we observed in isolates of various geographical origins and of different genera is a so far unique discovery”, summarizes Dr. Jörn Petersen.

Ocean’s Thirteen – a proteobacterium with 13 replicons
Decoding the entire genome of the newly described bacterium Marinibacterium anthonyi La 6 led to another striking observation. With one chromosome and twelve plasmids, the La 6 strain is the record-holder amongst Proteobacteria, while its genome organisation is similarly complex to that of plants and animals. With 6.8 million base pairs, Marinibacterium has the largest genome within the Rhodobacteraceae family. Amongst the dozen plasmids it is pLA6_12, the smallest element of a mere 7,053 base pairs, that turned out to be the most exciting. It contains a highly preserved backbone and an interchangeable cassette of genes that, under certain conditions, are essential for survival.

Unique nature of pLA6_12
The newly discovered pLA6_12 plasmid represents a previously unknown class of mobile genetic elements in the Roseobacter-group. Microorganisms use these elements to exchange DNA via horizontal gene transfer in an uncomplicated and timely manner. Analyses of the present study have shown that the now identified type of plasmid is found solely in the marine Roseobacter-group. The unique feature is that the pLA6_12 plasmid occurs in identical form in three different species of Rhodobacteraceae. Even though the exchange of plasmids forms the basis for the emergence of multi-resistant hospital germs, the natural transfer of a 100 percent identical plasmid across species boundaries has – surprisingly enough – never been described before. The present paper delivered experimental proof that the pLA6_12 plasmid can increase the tolerance to highly toxic chromate by a factor of 20, thereby allowing the bacterial host to survive in severely contaminated habitats. 

Hidden talents: spotlight on Roseobacter
Bacteria of the Roseobacter-group belong to Alphaproteobacteria, more specifically the Rhodobacteraceae family. They represent one of the most common marine bacterial groups and are found throughout the oceans of the world. Despite playing a pivotal role in the global carbon and sulphur cycles, they have made the shift into the research spotlight less than a decade ago. Their exceptionally versatile metabolism has led them to increasingly come to the fore of science, as they offer a high potential for biotechnological applications.
The Leibniz-Institute DSMZ is a proud partner of the collaborative research centre “Roseobacter (TRR 51)”, which is supported by the German Research Foundation (DFG) with the expressed goal of reaching a systems biology understanding of this major group of globally important marine bacteria. “Researching the Roseobacter-group has already given us fundamental answers to questions of evolutionary biology. But beyond new findings on the genetic organisation of the bacteria, we also expect to garner biotechnologically useful results”, explains Dr. Jörn Petersen.  

Literature
A marine plasmid hitchhiking vast phylogenetic and geographic distances. Petersen J, Vollmers J, Ringel V, Brinkmann H, Ellebrandt-Sperling C, Spröer C, Howat AM, Murrell JC, Kaster AK. Proc Natl Acad Sci U S A. 2019 Sep 23. pii: 201905878. doi: 10.1073/pnas.1905878116. [Epub ahead of print]