Expression of threonine synthase from Solanum tuberosum L. is not metabolically regulated by photosynthesis-related signals or by nitrogenous compounds

Although the control of carbon fixation and nitrogen assimilation has been studied in detail, little is known about the regulation of carbon and nitrogen flow into amino acids. In this paper the isolation of a cDNA encoding threonine synthase is reported (TS; EC 4.2.99.2) from a leaf lambda ZAP II-library of Solanum tuberosum L. and the transcriptional regulation of the respective gene expression in response to metabolic changes. The pattern of expression of TS by feeding experiments of detached petioles revealed that TS expression is regulated neither by photosynthesis-related metabolites nor by nitrogenous compounds. The present study suggests that the regulation of the conversion of aspartate to threonine is not controlled at the transcript level of TS. The nucleotide and deduced amino acid sequences of potato TS show homology to other known sequences from Arabidopsis thaliana and microorganisms. TS is present as a low copy gene in the genome of potato as demonstrated in Southern blot analysis. When cloned into a bacterial expression vector, the cDNA did functionally complement the Escherichia coli mutant strain Gif41. TS transcript was found in all tissues of potato and was most abundant in flowers and source leaves.