When protein sequences encoded by archaeal genomes are used to search protein sequence databases for similar sequences, some sequences have their best match with sequences from eubacteria. Kanamycin, streptomycin and tetracycline resistance genes found in gram negative and gram positive bacteria are essentially identical in nucleotide sequence. A plasmid, RSF1010, can successfully be conjugated from E.coli to cyanobacteria, actinomycetes, Agrobacterium and other bacteria. Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes.
Genes can move across species barriers by conjugation.
Symbiotic relationships may play a role in cross kingdom transfers of genes. The wide transfer of genes that must have occurred naturally weakens the argument that human intervention through genetic engineering is placing genes where they have never been before. Further Information Bacteria of the genus Agrobacterium have developed a special mechanism that allows transfer of genes from the bacterium to higher plant chromosomes.