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Truncated genes and flower colour evolution (Function and evolution of a transcriptional regulator of petal pigmentation from Antirrhinum)

glover3This project aims to characterise the function and evolution of a gene we have called AmMYB11s, isolated from Antirrhinum majus. The protein encoded by AmMYB11s has significant sequence similarity to AtMYB11 and AtMYB12 of Arabidopsis, two proteins involved in the regulation of flavonoid synthesis. Flavonoids have many roles in plants, but in Antirrhinum they are often the early steps in the production of pigments which colour the petals and make them attractive to pollinating bumblebees. However, AtMYB11s has a significantly different sequence from those of AtMYB11 and AtMYB12. Although the DNA sequences are extremely similar, a frame shift has introduced a stop codon into the AmMYB11s sequence. Insertion of 7 DNA bases into the cDNA would replace the missing amino acids and allow translation to continue in the same frame, regenerating an amino acid sequence very similar to that of AtMYB11 for the whole length of the protein.

MYB proteins function as transcriptional regulators. The sequence of the AmMYB11s gene and its predicted protein product suggest that the protein functions as a dominant negative, since the stop codon prevents translation of the region encoding the transcriptional activation domain. DNA binding proteins with transcriptional activation domains usually initiate or enhance transcriptional initiation from specific promoters. However, DNA binding proteins without transcriptional activation domains may function as competitors, binding promoter sequences without the ability to activate transcription. Thus the presence of such a dominant negative in a cell as well as a transcriptional activator may result in less mRNA of the target gene being produced than if only the transcriptional activator were present. Such systems may be used by plants to regulate the precise level of expression of transcription factor target genes. This ability to control the level of secondary metabolite production in response to environmental changes may have significantly enhanced the success of flowering plants and contributed to their extremely rapid radiation.

The duplication or copying error which led to the evolution of AmMYB11s did not occur in Antirrhinum majus itself. The same cDNA has been isolated and sequenced from two other species of Antirrhinum, A. pulverulenthum and A. braun-blanquetii. However, there is no similar sequence in the genome of Arabidopsis. Arabidopsis and Antirrhinum are very distant from one another in the Angiosperm family tree, with Antirrhinum in the order Lamiales, in the Euasterids, while Arabidopsis is placed in the Brassicales, in the Eurosids. It is therefore likely that the AmMYB11s sequence evolved after the divergence of the Rosids and the Asterids, giving the Asterids potentially a greater degree of flexibility in the regulation of their secondary metabolites. This hypothesis can be tested by investigating the distribution of the AmMYB11s sequence in other members of the Asterids, and in the Rosids, in order to establish where it first appeared.

This project would contain three main strands - (a) functional analysis of AmMYB11s through the production of transgenic plants misexpressing the gene and a screen for a mutant containing a transposon insertion, (b) identification of related sequences within Antirrhinum for investigation of competitive interactions, (c) identification of AmMYB11s-like sequences in other Asterid and Rosid lineages to investigate the evolution of the truncated protein, combined with tests of function in some of the genetically amenable species.

References

  1. Schwinn, K., J. Venail, Y. Shang, S. Mackay, V. Alm, E. Butelli, R. Oyama, P. Bailey, K. Davies, and C. Martin. (2006). A small family of MYB-regulatory genes controls floral pigmentation intensity and patterning in the genus Antirrhinum. Plant Cell 18:831–851.