Summary: Homoserine kinase, the product of the gene, catalyses an obligatory step of threonine biosynthesis. In unlike inactivation of the previously identified gene does not result in threonine auxotrophy. A new gene, named was isolated that, when expressed in mutant strains, results in complementation of the mutant phenotype. In threonine auxotrophy is observed only when both and are simultaneously inactivated. Thus, encodes a protein with an homoserine-kinase-like activity. Surprisingly, overexpression allows complementation of serine auxotrophy of and mutants. These mutants are affected in the phosphoserine phosphatase protein, an enzyme involved in serine biosynthesis. Comparison analysis revealed sequence homology between ThrH and the SerB proteins from different organisms. This could explain the phosphoserine phosphatase activity of ThrH when overproduced. ThrH differs from the protein encoded by the gene which was identified in Thus, two SerB-like proteins co-exist in a situation also found in


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