Characterization of thiI, a new gene involved in thiazole biosynthesis in Salmonella typhimurium

Thiamine pyrophosphate (TPP) is a required cofactor in Salmonella typhimurium that is generated de novo by the condensation of 4-amino-5-hydroxymethyl pyrimidine (HMP) pyrophosphate and 4-methyl-5-(b-hy-droxyethyl)-thiazole (THZ) monophosphate. The THZ and HMP moieties are independently synthesized, and labeling studies have demonstrated probable metabolic precursors to both. We present herein the initial characterization of thiI, a gene required for THZ synthesis. We show that thiI is a 1,449-bp open reading frame located at minute 9.65 on the S. typhimurium chromosome and that it encodes a 483-amino-acid protein with a predicted molecular mass of 55 kDa. Unlike genes in the thiamine biosynthetic operon at minute 90, thiI is not transcriptionally regulated by TPP. Thiamine pyrophosphate (TPP) is a required cofactor for a number of well-characterized enzymes (e.g., a-ketoglutarate dehydrogenase, pyruvate dehydrogenase, and transketolase). Although several recent studies of Saccharomyces cerevisiae (20– 23), Escherichia coli (2, 28), and Salmonella typhimurium (9, 25, 29) have furthered our understanding of the synthesis of this vitamin, the genetic and biochemical characterization of the bio