Identification of genes encoding the folate- and thiamine-binding membrane proteins in Firmicutes

Genes encoding high-affinity folate- and thiamine-binding proteins (FolT, ThiT) were identified in the Lactobacillus casei genome, expressed in Lactococcus lactis, and functionally characterized. Similar genes occur in many Firmicutes, sometimes next to folate or thiamine salvage genes. Most thiT genes are preceded by a thiamine riboswitch. The folate and thiamine transport systems of Lactobacillus casei were partially characterized 30 years ago by Henderson and colleagues (8, 9, 11, 12). These systems were shown to involve two small membrane proteins for specific substrate binding—one for folate and the other for thiamine—as well as an uncharacterized component shared by both systems. To identify genes encoding the binding proteins (FolT and ThiT), we used the AACompIdent tool on the ExPASy server (27) to search the L. casei (strain ATCC 334) genome for open reading frames with amino acid compositions and molecular masses matching those published for FolT an