University of Pittsburgh
June 1, 2000

lateral Gene Transfer the Primary Means of Bacterial Evolution, Says Pitt Researcher

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PITTSBURGH, May 31 -- Gene-swapping, or lateral gene transfer, is the primary means of bacterial evolution, and not an ancillary method as had been previously thought, according to a paper cowritten by a University of Pittsburgh professor in the May 18 edition of the journal Nature.

University of Pittsburgh Biology Professor Jeffrey G. Lawrence and colleagues Howard Ochman at the University of Arizona and Eduardo A. Groisman of the Washington University School of Medicine found extensive recent lateral gene transfer in each of the bacteria studied. The researchers also explained how to determine which DNA sequences have been transferred laterally, and if the transfers were recent.

"Bacterial chromosomes are mosaics of ancestral and laterally acquired sequences," said Lawrence. "Scientists have known for years that lateral gene transfer was one means of bacterial evolution, but until now it was assumed to be a secondary factor. But we've shown that, in fact, it is the primary method, ahead of gene mutation of ancestral DNA."

Sequences that are new to a bacterial genome retain the sequence characteristics of the donor genome and thus can be distinguished from ancestral DNA. Also, the regions adjacent to laterally transferred genes often contain traces of the sequences affecting their integration, such as remnants of the replaced gene, or are in sites known to facilitate gene transfer.

Bacteria can acquire gene sequences in three ways: conjugation, which is physical contact between donor and recipient cells; transformation, in which the bacteria pick up "naked" DNA—DNA which is no longer part of another organism; or transduction, in which a bacteriophage (bacterial virus) that has replicated itself in the donor microorganism and packaged random DNA fragments.

Conjugation and transformation both allow gene transfer not only between distantly related organisms, but between domains—for instance, a bacteria might pick up genes from a plant or animal. As an example, E. coli, one of the bacteria studied, has substantial portions of DNA transferred from mammals.

The only requirement for successful acquisition is that the newly acquired DNA sequence must be beneficial to the recipient organism. Pathogenic and antibiotic resistant gene sequences are easily transferred.

Lawrence said he marvels at the dramatic changes possible for bacteria when they acquire new gene sequences.

"Lateral genetic processes result in extremely dynamic genomes in which substantial amounts of DNA are introduced into, and deleted from, the chromosome and have effectively changed the ecological and pathogenic character of bacterial species," said Lawrence. "Bacteria display extraordinary variation in their metabolic properties, cellular structures, and lifestyles."

"A bacteria that once survived on organic matter might acquire a DNA sequence that would allow it to 'eat' metal, instead," said Lawrence. "That radical of a change is unheard of in other organisms."

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