6-thioguanine biosynthesis in Erwinia species
Erwinia amylovora produces a compound with an absorbance maximum at 340 nm that forms a yellow colored complex with copper. This compound was identified as 6-thioguanine, a guanine analogue that is used in chemotherapy and the treatment of inflammatory bowel disease. Synthesis of 6-thioguanine could be linked to five genes common in Erwinia genomes, but missing in related genera. Expression of Erwinia tasmaniensis genes tgsA-D in Escherichia coli was sufficient for heterologous production of 6-thioguanine. Transfer of the four biosynthetic E. tasmaniensis genes did not enhance resistance of E. coli against 6-thioguanine. Bacterial and synthetic 6-thioguanine have a strong growth inhibitory effect on many bacteria, such as E. coli or a number of Pantoea agglomerans isolates. While this inhibition of competing species might provide an advantage to Erwinia, further biological functions of 6-thioguanine cannot be excluded. A direct link between 6-thioguanine synthesis of E. amylovora and its pathogenicity was not observed, as two 6-thioguanine negative mutants produced as severe symptoms on apple and pear shoots as did producing strains.