Phospholipid biosynthesis commences with the acylation of glycerol-3-phosphate (G3P) to form 1-acyl-G3P. This step is catalyzed by the PlsB protein in Escherichia coli. The gene encoding this protein has not been identified, however, in the majority of bacterial genome sequences, including that of Bacillus subtilis. Recently, a new two-step pathway catalyzed by PlsX and PlsY proteins for the initiation of phospholipid formation in Streptococcus pneumoniae has been reported.
In B. subtilis, 271 genes have been reported to be indispensable, when inactivated singly, for growth in LB medium. Among these, 11 genes encode proteins with unknown functions. As part of a genetic study to identify the functions of these genes, we show here that the B. subtilis ortholog of S. pneumoniae PlsY, YneS, is required for G3P acyltransferase activity, together with PlsX. The B. subtilis genome lacks plsB, and we show in vivo that the PlsX/Y pathway is indeed essential for the growth of bacteria lacking plsB. Interestingly, in addition to plsB, E. coli possesses plsX and the plsY ortholog, ygiH. We therefore explored the functional relationship between PlsB, PlsX and YgiH in E. coli, and found that plsB is essential for E. coli growth, indicating that PlsB plays an important role in 1-acyl-G3P synthesis in E. coli. We also found, however, that the simultaneous inactivation of plsX and ygiH was impossible, revealing important roles for PlsX and YgiH in E. coli growth.
Both plsX and yneS are essential for 1-acyl-G3P synthesis in B. subtilis, in agreement with recent reports on their biochemical functions. In E. coli, PlsB plays a principal role in 1-acyl-G3P synthesis and is also essential for bacterial growth. PlsX and YgiH also, however, play important roles in E. coli growth, possibly by regulating the intracellular concentration of acyl-ACP. These proteins are therefore important targets for development of new antibacterial agents.