The maize mutant defective endosperm-B18 (de (*)-B18), which is recessive to its wildtype, accumulates substantially less dry matter in the endosperm than its normal counterpart. Both free and bound indole-acetic acid (IAA) content has been measured at 5 different developmental stages. In endosperm tissue, the level of IAA is at least 15 times lower in the mutantde (*) -B18 than in the wildtype. The situation found in the diploid tissues is somewhat different: in the mature embryo the level of total IAA is lower in the mutant than in the wildtype, while in 4-day old seedlings the level of total IAA is, to some degree, similar in both genotypes. Naphthalene-acetic acid (NAA), a stable synthetic auxin which mimics IAA in its biochemical effects, is able to normalize the seed weight of the mutant when applied to developing grains. The results favor the conclusion that in maize endosperm the mutationde (*) -B18 is involved in IAA metabolism.

Maize endosperms accumulate during development a large amount of storage proteins (zeins). The rate of zein accumulation is under the control of several regulatory genes. Two of these, the opaque-2 and opaque-6 mutants, lower the zein level, thus improving the nutritional quality of maize meals. An endosperm protein of Mr 32 000 (b-32) appears to be correlated with the zein level. The b-32 protein is encoded by the opaque-6 gene which, in turn, is activated by opaque-2. We report the purification, amino-acid composition and peptide map of b-32 protein. Furthermore we demonstrate that the protein exists as a monomer likely located in the soluble cytoplasm. As a step towards the isolation of a complementary-DNA clone for b-32 protein, the purification of its corresponding mRNA is described.

No abstract

myb7 mRNA is present in rice in spliced and unspliced forms, splicing being enhanced by anoxia. The protein (Mybleu) encoded by the unspliced mRNA is composed of an incomplete Myb domain followed by a leucine zipper; however, it lacks canonical sequences for DNA binding, transcriptional activation, and nuclear localization. We show here that in transiently transformed tobacco protoplasts, Mybleu is able to enhance the transcriptional activity of the maize leucine zipper Opaque2 on its target b32 promoter. The Mybleu transactivation effect is strictly dependent on the presence of Opaque2 and is driven by Mybleu-Opaque2 heterodimers. Mybleu is located in the nucleus, both in rice and in transformed tobacco protoplasts. In rice, the protein is expressed in regions corresponding to undifferentiated cells of roots and coleoptiles. Therefore, myb7 mRNA encodes, depending on its splicing, two transcription factors belonging to separate classes. One of them, Mybleu, has novel structural characteristics, suggesting the existence of new mechanisms acting in the activation of transcription.Mybs are a family of transcription factors widely represented in viruses, insects, mammals, and plants. The common feature of Myb factors is the presence of a conserved domain consisting of imperfect repeats of 50 -53 amino acids, called "tryptophan clusters" because of the highly conserved tryptophan residues involved in stabilizing the structure of the DNA binding domain (1). In plants, myb genes are present as large families (6 -100 members) involved in the control of a wide range of biochemical pathways (2-6), including responses to biotic and abiotic stresses (7-13).Screening a cDNA library from anaerobically grown rice coleoptiles, we isolated myb7, a cDNA derived from a Mybencoding unspliced mRNA (8). Some features of myb7 sequence indicate that it may be post-transcriptionally regulated; in particular, two unspliced introns are present in positions that are conserved with respect to other plant myb genes. Both spliced and unspliced myb7 mRNAs are present in vivo (7).In aerobically grown rice roots, we observed a higher proportion of unspliced myb7 RNA, with respect to the spliced form. The spliced form is instead predominant in rice roots during anoxia, a stress situation that generally inhibits splicing (14). These data suggest that post-transcriptional regulation controls the ratio between myb7 unspliced and spliced forms (7). This hypothesis is supported by the observation that the 5Ј region of the first intron can direct the synthesis of an in-frame leucine zipper, a functional domain present in several transcription factors (15-18).The putative polypeptide encoded by the unspliced myb7 mRNA consists of an incomplete Myb domain followed by the leucine zipper: we therefore named it Mybleu. Mybleu does not have the characteristics of a functional transcription factor. Indeed, it should be unable to bind DNA because of the presence of an incomplete Myb repeat. Moreover, neither a putative transcriptional activator region nor a c...

No abstract