The opportunistic bacterium Pseudomonas aeruginosa synthesizes significant amounts of an additional phospholipid, identified as 2' alanyl-phosphatidylglycerol (A-PG), when exposed to acidic growth conditions. At pH 5.3 A-PG contributed up to 6% to the overall lipid content of the bacterium. Sequence analysis of P. aeruginosa revealed open reading frame PA0920 showing 34% sequence identity to a protein from Staphylococcus aureus involved in tRNA-dependent formation of lysyl-phosphatidylglycerol. The P. aeruginosa deletion mutant DeltaPA0920 failed to synthesize A-PG. Heterologous overproduction of PA0920 in Escherichia coli resulted in the formation of significant amounts of A-PG, otherwise not synthesized by E. coli. Consequently, the protein encoded by PA0920 was named A-PG synthase. The enzyme was identified as an integral component of the inner membrane. The protein was partially purified by detergent solubilization and subjected to an in vitro activity assay. tRNA(Ala)-dependent catalysis was demonstrated. Transcriptional analysis of the corresponding gene in P. aeruginosa using lacZ reporter gene fusion under various pH conditions indicated a 4.4-fold acid-activated transcription. A phenotype microarray analysis was used to identify further conditions for A-PG function.