Further studies on zein gene structure

A series of new zein cDNA clones obtained by rescreening the previously constructed cDNA libraries were identified on the basis of cross- and differential-hybridization to previously characterized clones (Viotti et al., EMBO J. 1, 1982). The nucleotide and deduced amino acid sequences of the new clones were compared both within them and with respect to the already published sequences of zein genomic and cDNA clones.

On the basis of the four domains present in zein polypeptides, signal peptide, head piece (amino terminus), repeat block structure and tail piece (carboxyl terminus), at least four prototype sequences can be identified that do not necessarily correspond to the length of the various polypeptides detected by SDS-PAGE. As the comparison was carried out within sequences all belonging to the maize inbred W64A, the mutations found in the various clones of each prototype represent really different genes. This allows a better classification and a definition of the effects of the mutations on the various zein domains of the polypeptides belonging to each prototype. The comparison at the amino acid level among members belonging to each prototype indicates that three domains, signal, head and tail, were strongly conserved in sequence while the number, length and a.a. sequences of the blocks may vary extremely. In turn a gene can be assigned to a given prototype simply on the basis of the a.a. sequence of the three conserved regions even though its coding capacity may vary in length. This was found particularly true for cDNA clones coding for the so-called high molecular weight zein polypeptides, defined in our case by the M1 prototype (Viotti et al., EMBO J. 1, 1982). Among these, two independently isolated clones show a terminator codon twenty triplets upstream from the physiological one, resulting in truncated polypeptides and thus phenotypically belonging to the light chain zein class. A similar prematurely terminated gene has been found by sequencing a zein genomic clone (Spena et al., EMBO J. 1, 1982).

G. Cairo, E. Sala and A. Viotti
 
 

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