In the three domains of life, three biosynthetic pathways are known for putrescine. The first route is conversion of ornithine to putrescine by ornithine decarboxylase (ODC: SpeC), the second route is the conversion of arginine to agmatine by arginine decarboxylase (ADC: SpeA), followed by the conversion of agmatine to putrescine by agmatine ureohydrolase (AUH: SpeB), and the third route is the conversion of agmatine to N-carbamoylputrescine by agmatine deiminase (agmatine iminohydrolase, AIH), followed by the conversion of N-carbamoylputrescine to putrescine by N-carbamoylputrescine amidohydrolase (NCPAH). An extreme thermophile, Thermus thermophilus produces putrescine, although this bacterium lacks homologs for putrescine synthesizing pathways, such as ODC, AUH, AIH and NCPAH. To identify genes involved in putrescine biosynthesis in T. thermophilus, putrescine biosynthesis was examined by disruption of a predicted gene for agmatinase (agmatine ureohydrolase), or by using purified enzyme. It was found that arginase (TTHA1496) showed an agmatinase activity utilizing agmatine as a substrate. These results indicate that this bacterium can use arginase for putrescine biosynthesis. Arginase is a major contributor to putrescine biosynthesis under physiological conditions. The presence of an alternative pathway for converting agmatine into putrescine is functionally important for polyamine metabolism supporting survival at extreme environments.
Keywords: Thermus thermophilus; agmatine; aminopropylagmatine ureohydrolase; arginase; polyamine metabolism; putrescine.
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