Involved in the breakdown of putrescine via the biosynthesis of gamma-L-glutamylputrescine. Not required for glutamine synthesis (PubMed:34439822). Is not able to compensate for the loss of glutamine synthetase (PubMed:32296946). ATP + L-glutamate + putrescine = ADP + gamma-L-glutamylputrescine + H(+) + phosphate Binds 2 Mg(2+) ions per subunit. Amine and polyamine degradation; putrescine degradation; 4-aminobutanoate from putrescine. Dodecamer. Expression is induced in complex medium (CM) in which putrescine is both nitrogen (N) and carbon (C) source, in defined media (DM) with putrescine as the N source and glucose as the C source, DM with ammonium or nitrate as the N source and glucose as the C source, and in nitrogen starvation media (NS) without the N source and glucose as the C source (at protein level). Not expressed in CM in which yeast extract is both the N and the C source (at protein level). Constitutively expressed at different growth phases in CM in which yeast extract is both the N and the C source and in DM with ammonium or nitrate as the N source and glucose as the C source. Also constitutively expressed in NS regardless of the nitrogen deficiency time (48, 96 or 120 hours). Expression may be regulated at post-transcriptional level (PubMed:34439822). Expression is down-regulated in CM with 40 mM glutamine in the mid-exponential phase after nitrogen starvation for 72 hours (PubMed:32296946). No glutamate-putrescine ligase activity in complex medium (CM) in which putrescine is both nitrogen (N) and carbon (C) source, in defined media (DM) with putrescine as the N source and glucose as the C source nor in DM with nitrate as the N source and glucose as the C source. Belongs to the glutamine synthetase family.