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BpsA (N(4)-bis(aminopropyl)spermidine synthase) is a single-module non-ribosomal peptide synthase (NRPS) (also see non-ribosomal peptide (NPR)) located in the cytoplasm^[1] responsible for the process of creating branched-chain polyamines,^[1] and producing spermidine and spermine.^[2] It has a singular ligand in its structure involved with Fe3+ and PLIP interactions.^[3] As seen by its EC number, it is a transferase (2) that transfers an alkyl or aryl group other than methyl groups (5) (2.5.1).^{[citation needed]} BpsA was first discovered in the archaea Methanococcus jannaschii^[4] and thermophile Thermococcus kodakarensis ^[2] and since then has been used in a variety of applications such as being used as a reporter, researching phosphopantetheinyl transferase (PPTase), and for NRPS domain recombination experiments it can be used as a model.^[5] Both (hyper)thermophilic bacteria and euryarchaeotal archaea seem to conserve BpsA and orthologs as branches chains polyamines are crucial for survival.^[6] There is also a second type of BpsA also known as Blue-pigment indigoidine synthetase that produces the pigment indigoidine and is found in organisms like Erwinia chrysanthemi.^[7] However, not much seems to be known about this variant except that it is a synthase, and it does not yet appear to be classified under an EC number.

In thermophiles, BpsA converts N4-aminopropylspermidine to N4-bis(aminopropyl)spermidine.^[2] In this pathway, aminopropyltransferase and ureohydrolase turn N1-aminopropylagmatine to agmantine and synthesize spermidine and spermine.^[2] Spermine and spermadine are utilized in a variety of pathways including macromolecule production, apoptosis and proliferation equilibrium, and the induction of differentiation in cells.^[8] Long Linear polyamines (such as ones found in TK-BpsA made of up spermine and spermidine) help stabilize DNA.^[8] Denaturation could possible occur at high temperatures, making the stabilization crucial for organisms that thrive here. If an organism cannot stabilize its DNA, it cannot survive. TK-BpsA is a BpsA found in the archaeon Thermococcus kodakarensis and is used to study this pathway more in depth.^[9] It is also a ternary complex.^[10] There are a few active sites that include polyamine spermidine/spermine synthases, and loop-closures occur upon the binding of spermidine, and a catalytic center made of a Gly-Asp-Asp-Asp motif which contains reactive secondary amino group of the substrate polyamine and a sulfur atom of the product 5ʹ-methylthioadenosine with Asp 159.^[9] The enzyme proves itself to be important to thermophiles as it supports growth under high-temperature conditions.^[1] In this system, the C-Terminal is a flexible region of branched-chain polyamine synthase facilitates substrate specificity and catalysis.^[10] This C-terminal region recognizes acceptor proteins for the enzyme and gain their flexibility from aspartate/glutamate residues.^[10] The flexibility itself is promoted by a ping-pong Bi-Bi mechanism that occurs when temperatures are high.^[10]