Proposed models of polyamines metabolism and aIF5A in Hfx. volcanii. (a) Predicted pathways of polyamines synthesis from arginine substrate. Dashed arrows indicate proposed reaction; solid arrow indicates known reaction. (b–d) Proposed substrates of DHS. HVO_1958 encodes a potential arginine decarboxylase; HVO_2299 an agmatinase-like; HVO_0255 a spermidine synthase; HVO_B0182 a spermidine synthase or a homospermidine synthase; and HVO_2297 a potential deoxyhypusine synthase.