RESUMO
The sunflower sf21C gene is a member of a small plant gene family related to the human N-myc downstream-regulated gene family (NDRG) involved in stress and hormone responses, cell growth and differentiation. We have shown earlier that this sunflower gene is interrupted by nine introns and generates multiple mRNAs by alternative splicing of its primary transcript (Lazarescu et al. in Gene 374:77-86, 2006). In the present study, we have characterized 20 splice isoforms of this gene encoding 14 different protein variants. We show that these isoforms occur in distinct combinations in the various plant organs. Five identified variants were generated by splicing at novel sites, different from those classically recognized by U2- or U12-type spliceosomes. Twelve transcript variants contain premature termination codons (PTCs), which make them potential candidates for nonsense-mediated RNA degradation (NMD). Interestingly, all 12 transcripts are relatively abundant in at least one organ, suggesting that NMD is not a "default" mechanism for PTC-containing RNA transcripts generated by alternative splicing. These data indicate that alternative splicing of the sunflower sf21C primary transcript as well as the mRNA quality control mechanism by NMD are complex and regulated spatially and developmentally.
Assuntos
Processamento Alternativo/genética , Regulação da Expressão Gênica de Plantas/genética , Helianthus/genética , Proteínas de Plantas/genética , Estruturas Vegetais/genética , Sequência de Aminoácidos , Sequência de Bases/genética , Códon sem Sentido/genética , Éxons/genética , Helianthus/crescimento & desenvolvimento , Helianthus/metabolismo , Dados de Sequência Molecular , Especificidade de Órgãos/genética , Estruturas Vegetais/crescimento & desenvolvimento , Estruturas Vegetais/metabolismo , Isoformas de Proteínas/genética , Sítios de Splice de RNA/genética , Estabilidade de RNA/genéticaRESUMO
The SF21 proteins were originally identified in sunflower pollen and in the stigmatic and transmitting tissues of sunflower pistils [Kräuter-Canham, R., Bronner, R., Evrard, J.L., Hahne, G., Friedt, W. and Steinmetz, A., 1997. A transmitting tissue- and pollen-expressed protein from sunflower with sequence similarity to the human RTP protein. Plant Science 129, 191-202.]. They are polypeptides of about 350 amino acids showing limited but significant sequence similarities with the animal NDR/RTP family of proteins of yet unknown function. Based on genomic sequence information derived from BAC clones containing SF21-related sequences we have identified transcripts generated from three different, but highly related genomic copies: SF21C, SF21D and SF21E. A sequence analysis of SF21C transcripts amplified by RT-PCR using specific primer pairs revealed a complex splicing pattern producing a minimum of three splice variant forms of the protein, one of 355 residues, and two truncated proteins of 90 and 138 amino acids, respectively. One of these variants was detected only in styles from pollinated florets, indicating organ-specific splicing. Two other splice variants, identified for a related transcript, SF21D, generate proteins differing by an 8-residue extension at their C-terminus. This analysis of SF21 transcripts in sunflower supports already existing evidence that alternative splicing is complex and common in plants.