Involvement of two related porins, OprD and OpdP, in the uptake of arginine by Pseudomonas aeruginosa

The OprD family of specific porins in Pseudomonas aeruginosa comprises 19 members, some of which have been demonstrated to facilitate the uptake of specific compounds into the cell. The members of this family share considerable amino acid sequence similarity (46-57%), which is unusual among porin molecules. In this work, we sought to establish whether this sequence conservation was the basis for other shared aspects of this family. The transcriptional profiles of eight relatively well-characterized OprD homologs were assessed in cells grown on a variety of carbon compounds. The expression of these paralogous proteins correlated with their phylogenetic distribution into two subfamilies in that the three members of the OpdK subfamily were induced by their specific (organic acid) substrates while the five members of the amino-acid/peptide-specific OprD subfamily appeared to be constitutively expressed. Functional overlap with respect to arginine transport was observed between two members of the latter subfamily, the basic amino acid-specific porin, OprD, and the glycine-glutamate-specific porin, OpdP. The impact of this apparent functional redundancy on the genetic fitness of P. aeruginosa is discussed.