Investigation of the polyamine biosynthetic and transport capability of Streptococcus agalactiae : the non-essential PotABCD transporter

Polyamines constitute a group of organic polycations positively charged at physiological pH. They are involved in a large variety of biological processes, including the protection against physiological stress. In this study, we show that the genome of , a commensal bacterium of the intestine and the vagina and one of the most common agents responsible of neonate infections, does not encode proteins homologous to the specific enzymes involved in the known polyamine synthetic pathways. This lack of biosynthetic capability was verified experimentally by TLC analysis of the intracellular content of grown in the absence of polyamines. However, similar analyses showed that the polyamines spermidine, spermine and putrescine can be imported from the growth media into the bacteria. We found that all strains of possess the genes encoding the polyamine ABC transporter PotABCD. We demonstrated that these genes form an operon with , a gene involved in folate biosynthesis, , a gene involved in peptidoglycan biosynthesis, and with , a gene encoding a Cl /H antiporter involved in resistance to acid stress in . Transcription of the operon is induced by peroxide-induced oxidative stress but not by acidic stress. Spermidine and spermine were found to be inducers of transcription at pH 7.4 whereas putrescine induces this expression only during peroxide-induced oxidative stress. Using a deletion mutant of , we were nevertheless unable to associate phenotypic traits to the PotABCD transporter, probably due to the existence of one or more as yet identified transporters with a redundant action.