Biochemical characterisation of Nicotiana plumbaginifolia auxotrophs that require branched-chain amino acids.

The biochemical lesions in six amino acid-requiring auxotrophic lines of Nicotiana plumbaginifolia have been investigated, by means of feeding experiments with [(14)C] and unlabelled substrates, and enzyme analysis. Three of the lines require isoleucine for growth, are unable to synthesise 2-oxobutyrate in vivo and have no detectable threonine dehydratase (E.C.4.2.1.16) in vitro. The other three lines require (isoleucine + valine), accumulate [(14)C] dihydroxymethylvalerate when fed [(14)C]-L-threonine, and contain no detectable dihydroxyacid dehydratase (E.C.4.2.1.9).