Synthesis of Lectin-Like Protein in Developing Cotyledons of Normal and Phytohemagglutinin-Deficient Phaseolus vulgaris

The genome of the common bean Phaseolus vulgaris contains a small gene family that encodes lectin and lectin-like proteins (phytohemagglutinin, arcelin, and others). One of these phytohemagglutinin-like genes was cloned by L. M. Hoffman et al. ([1982] Nucleic Acids Res 10: 7819-7828), but its product in bean cells has never been identified. We identified the product of this gene, referred to as lectin-like protein (LLP), as an abundant polypeptide synthesized on the endoplasmic reticulum (ER) of developing bean cotyledons. The gene product was first identified in extracts of Xenopus oocytes injected with either cotyledonary bean RNA or LLP-mRNA obtained by hybrid-selection with an LLP cDNA clone. A tryptic map of this protein was identical with a tryptic map of a polypeptide with the same SDS-PAGE mobility detectable in the ER of bean cotyledons pulse-labeled with either [³H]glucosamine or [³H]amino acids, both in a normal and in a phytohemagglutinin-deficient cultivar (cultivars Greensleeves and Pinto UI 111). Greensleeves LLP has M_r 40,000 and most probably has four asparagine-linked glycans. Pinto UI 111 LLP has M_r 38,500. Unlike phytohemagglutinin which is a tetramer, LLP appears to be a monomer by gel filtration analysis. Incorporation of [³H]amino acids indicates that synthesis of LLP accounts for about 3% of the proteins synthesized on the ER, a level similar to that of phytohemagglutinin.     

Differences in expression between two seed lectin alleles obtained from normal and lectin-deficient beans are maintained in transgenic tobacco

Using Agrobacterium-mediated transformation, two genes for phytohemagglutinin-L (PHA-L), the lectin seed protein of the common bean Phaseolus vulgaris, were stably integrated into the tobacco genome. The two alleles for PHA-L, dlec2 and pdlec2, were obtained from a normal cultivar (Greensleeves) and a lectin-deficient cultivar (Pinto) respectively. In the bean embryos, the expression of dlec2 is 30 times greater than the expression of pdlec2. In the dlec2-transformed tobacco, PHA-L accumulated specifically in the seeds at the same stages as the tobacco seed storage proteins and was degraded after germination. PHA-L was found in the embryo, and at a 5–7 times lower concentration in the endosperm tissue of the mature tobacco seeds. No PHA could be detected in other parts of the plants. We conclude that the signals for temporal and spatial regulation of the dlec2 gene are present in the DNA fragment used for transformation. Transformation with the second PHA-L allele pdlec2 from the cultivar Pinto caused the accumulation of about 50 times less PHA-L in tobacco seeds when compared to dlec2. We conclude from analyzing the 5' sequences of dlec2 and Pdlec2 that the low expression phenotype of the Pdlec2 allele could be due to the absence or mutation of a cis-acting element carried by the dlec2 fragment.     

Expression of a bean storage protein 'phaseolin minigene' in foreign plant tissues

Using the phaseolin gene and its cDNA counterpart we constructed a mutant phaseolin gene lacking the five introns but retaining its natural 5' and 3' plant-regulatory sequences. This mutant phaseolin gene (minigene) was inserted into the Ti-plasmid of Agrobacterium tumefaciens strain 15955 which allowed its transfer and integration into the tobacco genome. Full-length and correctly initiated phaseolin mRNA was found among the poly(A)+RNA isolated from plant callus transformed with the minigene construction by using RNA-DNA hybridization and S1 nuclease mapping techniques. The presence of phaseolin polypeptides in soluble protein extracts from transformed tobacco tissues was confirmed by immunological methods. These results demonstrate that phaseolin gene introns and intron splicing are not a necessary requirement for biogenesis of stable phaseolin mRNA and that no alternative splice site was introduced by the removal of five introns.     

Pet111 Acts in the 5''-Leader of the Saccharomyces Cerevisiae Mitochondrial Cox2 mRNA to Promote Its Translation

PET111 is a yeast nuclear gene specifically required for the expression of the mitochondrial gene COX2, encoding cytochrome c oxidase subunit II (coxII). Previous studies have shown that PET111 activates translation of the COX2 mRNA. To map the site of PET111 action we have constructed, in vitro, genes coding for chimeric mRNAs, introduced them into mitochondria by transformation and studied their expression. Translation of a chimeric mRNA with the 612-base 5'-untranslated leader of the COX3 mRNA fused precisely to the structural gene for the coxII-precursor protein is independent of PET111, but does require a COX3 mRNA-specific translational activator known to work on the COX3 5'-leader. This result demonstrates that PET111 is not required for any posttranslational step. Translation of a chimeric mRNA with the 54-base 5'-leader of the COX2 mRNA fused precisely to the structural gene for cytochrome c oxidase subunit III was dependent on PET111 activity. These results demonstrate that PET111 acts specifically at a site in the short COX2 5'-leader to activate translation of downstream coding sequences.