and, not determined; 16 g N is generally regarded as equivalent to 100 g protein and g/16 g N as percentage by weight.

cKoshiyama (84) expressed as g/100 g protein.

"Bradley et al. (85) expressed as g/100 g protein.

"Ewart (86) expressed as g/100 g recovered amino acids.

'Misra and Mertz (87) expressed as g/100 g protein. The ethanol-soluble zein fraction contains mainly a-zeins and the ethanol-insoluble fraction ß-, y-, and S-zeins. Tryptophan is known to be absent from a-zeins, /3-zeins, and M, 27,000 y-zeins and present at about 1 mol % or less in M, 16,000 -^zeins and a-zeins (17). 3Ewart (88) expressed as g/100 g recovered amino acids.

"Singh and Sastry (89) expressed as g/16 g N. Data for alcohol-soluble hordeins only.

'WHO recommended levels (90) expressed as g/100 g protein. Histidine is regarded as essential for human children but not adults and is not listed in the WHO recommendations.

respect: aspartate kinase (AK), which catalyzes the first reaction in the pathway, and dihydrodipicolinate synthase (DHDPS), which controls the branch point leading to lysine (Fig. 2). Falco et al. (42) initially showed that a twofold increase in total lysine occurred in seeds of oilseed rape (canola) expressing a DHDPS gene from Corynebacterium and a fivefold increase occurred in soybean seeds expressing the same enzyme together with an AK gene from E. coli. More recent work, described in a patent application (43), has shown that expression of the DHDPS gene in maize under control of the embryo + aleurone-specific globulin-1 promoter resulted in increases in free lysine from 1.4 to 15-27% of total free amino acids, representing increases in total lysine from 2.3 to 3.6-5.3%. In contrast, no effect was observed when the same gene was expressed under control of the starchy endosperm-specific glutelin-2 promoter, which was thought to be due to increased lysine catabolism. It was concluded that down-regulation of lysine catabolism would be required in order to achieve accumulation of lysine in the maize starchy endosperm.

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