thrABC

thrABC is a conventional designation for a bacterial gene cluster that encodes the enzymes of the threonine biosynthesis pathway: thrA, thrB, and thrC. In many bacteria, thrA encodes a bifunctional enzyme with aspartokinase I activity and homoserine dehydrogenase I activity, while thrB encodes homoserine kinase and thrC encodes threonine synthase. Together, these enzymes channel aspartate into the amino acid threonine.

The sequence of reactions begins with aspartokinase (thrA), which catalyzes the formation of aspartyl phosphate. Homoserine dehydrogenase (also part of thrA) then reduces this intermediate to homoserine. Homoserine kinase (thrB) phosphorylates homoserine to O-phosphohomoserine, which is finally converted to threonine by threonine synthase (thrC). The pathway supplies threonine for protein synthesis and, in many organisms, also participates in broader amino acid metabolism.

Genomic organization and regulation of thrABC vary among species. thrA, thrB, and thrC can be co-regulated or arranged in an operon in some bacteria, while in others they are independently transcribed. The pathway is subject to feedback control by threonine, and in some organisms this regulation affects enzyme activity and gene expression levels. The bifunctional nature of thrA adds an additional layer of regulatory complexity.

Ecological and biotechnological relevance: threonine biosynthesis is essential for organisms that rely on endogenous production, making thrABC a focus of metabolic engineering aimed at boosting threonine yields or modulating flux through related pathways. Variants and rearrangements of thrABC reflect broader patterns of operon evolution and prokaryotic amino acid biosynthesis.