Extreme conservation of the psaA/psaB intercistronic spacer reveals a translational motif coincident with the evolution of land plants.
J Mol Evol
; 75(5-6): 184-97, 2012 Dec.
Article
em En
| MEDLINE
| ID: mdl-23192453
ABSTRACT
Although chloroplast transcriptional and translational mechanisms were derived originally from prokaryote endosymbionts, chloroplasts retain comparatively few genes as a consequence of the overall transfer to the nucleus of functions associated formerly with prokaryotic genomes. Various modifications reflect other evolutionary shifts toward eukaryotic regulation such as posttranscriptional transcript cleavage with individually processed cistrons in operons and gene expression regulated by nuclear-encoded sigma factors. We report a notable exception for the psaA-psaB-rps14 operon of land plant (embryophyte) chloroplasts, where the first two cistrons are separated by a spacer region to which no significant role had been attributed. We infer an important function of this region, as indicated by the conservation of identical, structurally significant sequences across embryophytes and their ancestral protist lineages, which diverged some 0.5 billion years ago. The psaA/psaB spacers of embryophytes and their progenitors exhibit few sequence and length variants, with most modeled transcripts resolving the same secondary structure a loop with projecting Shine-Dalgarno site and well-defined stem that interacts with adjacent coding regions to sequester the psaB start codon. Although many functions of the original endosymbiont have been usurped by nuclear genes or interactions, conserved functional elements of embryophyte psaA/psaB spacers provide compelling evidence that translation of psaB is regulated here by a cis-acting mechanism comparable to those common in prokaryotes. Modeled transcripts also indicate that spacer variants in some plants (e.g., aquatic genus Najas) potentially reflect ecological adaptations to facilitate temperature-regulated translation of psaB.
Texto completo:
1
Coleções:
01-internacional
Base de dados:
MEDLINE
Assunto principal:
Sequência Conservada
/
Evolução Molecular
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Embriófitas
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DNA Intergênico
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Complexo de Proteína do Fotossistema I
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Genes de Cloroplastos
Tipo de estudo:
Prognostic_studies
Idioma:
En
Ano de publicação:
2012
Tipo de documento:
Article