Table I.

β-Ketothiolase activity as a function of illumination

Enzyme activity for transgenic lines 4A and 4B and untransformed tobacco plants at the respective illumination periods are shown. Protein samples were obtained by grinding 1 g of leaf tissue in liquid nitrogen, followed by the addition of β-ketothiolase extraction buffer (100 mm Tris-HCl, pH 8.1, 50 mm MgCl₂, 5 mm BME) and homogenization. Total plant protein concentration was determined by Bradford assay. Ten microliters of crude plant extract was used per assay. β-Ketothiolase activity was measured spectrophotometrically at 304 nm for 60 s in the thiolysis direction. na, Not applicable.

Plant Line	Illumination Period Light/Dark	Enzyme Activity Units
	h	μmol/min
Untransformed	16 h/8 h	<0.0001
4A	16 h/8 h	0.215
4B	16 h/8 h	0.259
Untransformed	5 d of light	<0.0001
4A	5 d of light	0.247
4B	5 d of light	0.239
Purified bacterial β-ketothiolase	na	0.017