Maize beta-tubulins are encoded by a large multigene family with at least nine members, as determined by Southern blot analysis. Two expressed genes, represented by the beta 1 genomic clone and the beta 2 cDNA clone, were examined in this study. The two genes encode beta-tubulins which show 94% sequence identity at the amino acid level. Maize beta 1 transcript levels were highest in seedling root tip and tissue culture cells, which are both rapidly dividing tissues. No transcripts were detected in non-dividing leaf tissue. In contrast, beta 2 transcripts were present at relatively high levels in tissue culture cells and at lower levels in seedling root tip and leaf tissue. The electrophoretic mobility of the beta 2 polypeptide was examined in relation to the constellation of beta-tubulin polypeptides on two-dimensional gel western blots of a maize pollen total protein extract. No evidence for post-translational modification of the beta-tubulin polypeptides was found in pollen.

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We have isolated several Zea mays cDNAs encoding the 40S subunit ribosomal protein S14. In maize, this ribosomal protein is encoded by a small multigene family, at least three members of which are expressed. S14 transcript levels are highest in mitotically active tissues, such as seedling shoot, developing endosperm, and tassel primordia, and lowest in tissues with little cell division, such as mature leaf and root. Very little S14 RNA is present in pollen, suggesting that translation of pollen mRNAs during pollen germination uses preformed ribosomes. During kernel development, the highest levels of S14 transcripts in endosperm tissue are found at 10-12 days postpollination; S14 RNA levels decline continuously from this point onward. The period of maximal expression of the S14 ribosomal protein gene appears to precede the onset of storage protein synthesis and does not correlate with the reported times of increased nucleolar volume or genome amplification.

A methionine-rich 10 kDa zein storage protein from maize was isolated and the sequence of the N-terminal 30 amino acids was determined. Based on the amino acid sequence, two mixed oligonucleotides were synthesized and used to probe a maize endosperm cDNA library. A full-length cDNA clone encoding the 10 kDa zein was isolated by this procedure. The nucleotide sequence of the cDNA clone predicts a polypeptide of 129 amino acids, preceded by a signal peptide of 21 amino acids. The predicted polypeptide is unique in its extremely high content of methionine (22.5%). The maize inbred line BSSS-53, which has increased seed methionine due to overproduction of this protein, was compared to W23, a standard inbred line. Northern blot analysis showed that the relative RNA levels for the 10 kDa zein were enhanced in developing seeds of BSSS-53, providing a molecular basis for the overproduction of the protein. Southern blot analysis indicated that there are one or two 10 kDa zein genes in the maize genome.

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Maize glutamine synthetase cDNA clones were isolated by genetic selection for functional rescue of an Escherichia coli delta glnA mutant growing on medium lacking glutamine. The Black Mexican Sweet cDNA library used in this study was constructed in pUC13 such that cDNA sense strands were transcribed under the control of the lac promoter. E. coli delta glnA cells were transformed with cDNA library plasmid DNA, grown briefly in rich medium to allow phenotypic expression of the cDNAs and the pUC13 ampr gene, and challenged to grow on agar medium lacking glutamine. Large numbers of glutamine synthetase cDNA clones have been identified in individual 150-mm Petri dishes; all characterized cDNA clones carry complete coding sequences. Two cDNAs identical except for different 5' and 3' termini have been sequenced. The major open reading frame predicts a protein with an amino acid sequence that exhibits striking similarity to the amino acid sequences of the predicted products of previously sequenced eukaryotic glutamine synthetase cDNAs and genes. In addition, the maize glutamine synthetase cDNAs were shown to contain a 5' mini-ORF of 29 codons separated by 37 nucleotide pairs from the major ORF. This mini-ORF was shown not to be essential for the functional rescue of the E. coli delta glnA mutant. Expression of the cDNAs in E. coli is presumed to be due to the function of a polycistronic hybrid lac messenger RNA or translational fusions encoded by the pUC plasmids. Proteins of the expected sizes encoded by two different pUC clones were shown to react with antibodies to tobacco glutamine synthetase.