The deficiency of normal corn for the amino acids was recognized at the turn of the century by the studies of Osborne (1897, 1914), and Osborne and Mendel (1914). Improvement of the feeding qualities of normal corn was attempted by selection for higher crude protein in the corn kernel. In 1964, E. T. Mertz and co-workers reported that the opaque-2 gene changed the protein composition and increased the lysine content of corn endosperm. Protein mutants were isolated in the M3 generation after kernel treatment with combinations of radiation (gamma rays) and the chemical mutagens N-methylo-N-nitrosourea (MNUA) and sodium azide (SA). The mutant kernels analyzed were mature, air dried, and had been produced by self-pollination. Protein content (nitrogen x 6.25) was analyzed by the micro-Kjeldahl method. The amino acids in 100 mg of whole kernels were determined in an HD-1200E automatic amino acid analyzer.
The amino acid and protein contents of kernels are shown in Table 1. Our opaque mutant strains contained more protein than the normal strain (S-615) and opaque-2 strain (S-10), but less lysine (except mutant M-204-47) than the opaque-2 strain. The content of total protein was found to be increased by 1.4-3% when compared to the opaque-2 line. Mutant strains contained more histidine, arginine, aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, cysteine, valine, isoleucine, leucine, tyrosine, and phenylalanine, but less methionine. It is apparent that a new source of opaque mutant gene, which has a different biochemical characteristic than the opaque-2 mutant, has been induced by mutagens.
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