9.A.18.1.2
Bacteroid development protein, BacA; probably a peptide/bleomycin uptake transporter (Ardissone et al., 2011; Marlow et al., 2009). The bacterial BacA protein is essential for bacteroid differentiation in legumes producing nodule-specific cysteine-rich peptides (NCRs), which induce the terminal differentiation of the bacteria into bacteroids (Haag et al. 2013). This protein and others in this family are homologous to the membrane constituents of 3.A.1.203 family in the ABC superfamily in TCDB. Takes up phazolicin (PHZ), a peptide antibiotic (Travin et al. 2023). The frequency of spontaneous PHZ-resistant mutants in Sinorhizobium meliloti is below the detection limit because PHZ can enter S. meliloti cells through two promiscuous peptide transporters,, BacA (TC# 9.A.18.1.2) and YejABEF (TC# 3.A.1.5.49), which belong to the SLiPT (SbmA-like peptide transporter) and ABC (ATP-binding cassette) transporter families, respectively. The dual-uptake mode explains the lack of observed resistance acquisition because the simultaneous inactivation of both transporters is necessary for resistance to PHZ. Since both BacA and YejABEF are essential for the development of functional symbiosis of S. meliloti with leguminous plants, the unlikely acquisition of PHZ resistance via the inactivation of these transporters is further disfavored. A whole-genome transposon sequencing screen did not reveal additional genes that can provide strong PHZ resistance when inactivated. However, it was found that the capsular polysaccharide KPS, a novel putative envelope polysaccharide PPP (PHZ-protecting polysaccharide), as well as the peptidoglycan layer jointly contribute to the sensitivity of S. meliloti to PHZ, most likely serving as barriers that reduce the amount of PHZ transported inside the cell (Travin et al. 2023).