“…Targets of PII regulation include transcriptional regulators, enzymes, and transporters involved in nitrogen and carbon assimilation ( Burillo et al, 2004 ; Llácer et al, 2007 , 2008 ; Forcada-Nadal et al, 2018 ; Labella et al, 2020b ; Forchhammer et al, 2022 ). Cyanobacterial genomes always contain at least as many copies of glnB as of pipX ( Laichoubi et al, 2011 ) and in S. elongatus , a high PipX/PII ratio prevents growth ( Espinosa et al, 2009 , 2010 ; Laichoubi et al, 2012 ; Chang et al, 2013 ; Jerez et al, 2021 ; Llop et al, 2023 ) suggesting that a relatively high ratio of PII over PipX is required to counteract unwanted interactions with less abundant and/or low affinity PipX partners. So far, EngA is the best candidate for this predicted PipX binding protein.…”