The pab1 gene of Coprinus cinereus encodes a bifunctional protein for para-aminobenzoic acid (PABA) synthesis: implications for the evolution of fused PABA synthases

Abstract

The pab1 gene of the basidiomycete Coprinus cinereus encodes PABA synthase, necessary for para-aminobenzoic acid production. The C. cinereus protein is bifunctional with an N-terminal glutamine amidotransferase domain and a C-terminal chorismate amination domain. In most bacteria, these two functions are encoded in separate genes (e.g., pabA and pabB of E coli). Fused PABA synthases have so far been detected in actinomycetes, Plasmodium falciparum, fungi and Arabidopsis thaliana. Phylogenetic analysis shows that the fused PAB sequences form a tight group that also includes uncharacterized PabB homologues from several bacteria. Unfused bacterial PabA proteins group with the glutamine amidotransferase subunits of bacterial anthranilate synthases, independent of organismal systematics, indicating a complex and perhaps independent evolutionary origin. In contrast, unfused PabB group and fused PabA/B proteins form a monophyletic group on a branch separate from the chorismate amination subunits of anthranilate synthases, probably reflecting a need for recognition of different positions in the common substrate chorismate.