Vicki L. Chandler. Professor, Department of Plant Sciences, Ph. D., UC - San Francisco. Genetic mechanisms controlling gene expression in different cell types and at particular times during the development.. Chromatin-level control of gene expression.

 

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Vicki L. Chandler, Professor, Department of Plant Sciences. Ph.D. UC - San Francisco.

We investigate the control of gene expression in different cell types and at different developmental stages by studying the regulated expression of the genes required for the biosynthesis of the purple anthocyanin pigments in maize. In higher plants, this system provides one of only a few examples of known regulatory proteins, which coordinately regulate a number of target genes that are cloned and characterized. Environmental and physiological processes have a major impact on the regulation of this pathway as well. We have focused our attention on the b regulatory gene, which encodes a basic-helix-loop-helix transcription factor. Anthocyanin pigments are produced in tissues in which there is an overlap of expression of b together with either c1 or p11, which encode functionally equivalent myb transcription factors. More than thirty naturally occurring alleles of b exist, which show distinct tissue and developmental expression. Certain alleles of b also undergo paramutation--a heritable alteration in gene expression promoted by the presence of two specific alleles in the same plant. Our long-term goals are to understand how the B protein coordinately regulates the pathway, how environmental, developmental and physiological signals are used to control b expression, and the molecular basis of paramutation.

Molecular and genetic characterization of several b alleles with differential expression patterns indicate that they are ~98% identical in the coding region, but have major insertions and other rearrangements in the 5í flanking regions, where tissue-specific regulation maps. Transgenic analyses are being used to identify key regulatory sequences, and biochemical analyses and genetic screens are being used to identify factors regulating the expression. We have identified a new regulator of the anthocyanin pathway, pac1.  Experiments are in progress to clone pac1 and determine its role in anthocyanin gene regulation.

Paramutation at b occurs when a dark purple plant containing the B-I allele is heritably changed to B’ (light purple) when B-I and B’ are together in a heterozygote. We have demonstrated that the heritable change is accompanied by a ten- to twenty-fold reduction in b transcription. In spite of this dramatic change in transcription, extensive sequencing and restriction mapping have not detected structural or DNA methylation alterations in and around b that correlate with paramutation. Recent experiments are consistent with heritable changes in chromatin structure being involved in establishing and maintaining distinct transcription states.  Currently experiments are to characterize the minimal sequences required for paramutation, which map within 50-85 kbp upstream of the transcription initiation site.  We have also identified several mutations required for the establishment and maintenance of paramutation.  We have shown that these mutants also activate silent transposable elements, suggesting that the wild type proteins are required both for paramutation and the silencing of transposons.  Experiments are in progress to clone the genes represented by these mutations and determine their role in paramutation

Selected Publications:

Patterson, G.I. Thorpe, C.J. and V.L. Chandler. Paramutation, an allelic interaction, is associated with a stable and heritable reduction of transcription of the maize b regulatory gene. Genetics 135:881-894 (1993).

Patterson, G.I., K.M. Kubo, T. Shroyer, and V.L. Chandler. Sequences required for paramutation of the maize b gene map to a region containing the promoter and upstream sequences. Genetics 140:1389-1406 (1995).

Hollick, J.B., G.I. Patterson, E.H. Coe, Jr., K.C. Cone and V.L. Chandler. Allelic interactions heritably alter the activity of a metastable maize pl allele. Genetics 141:709-719 (1995).

Sainz, M.B., S.A. Goff and V.L. Chandler. Extensive mutagenesis of a transcriptional activation domain identifies single hydrophobic and acidic amino acids important for activation in vivo. Molecular Cell Biology 17(1):115-122 (1997).

Sainz, M.B., Grotewold, E. and V.L. Chandler. Evidence for direct activation of an anthocyanin promoter by the maize Cl protein and comparison of DNA binding by related Myb domain proteins, in press. Plant Cell 9:611-625 (1997).

Lesnick, M.L. and V.L. Chandler.  Activation of the maize anthocyanin gene a2 is mediated by an element conserved in many anthocyanin promoters.  Plant Physiol 117: 437-445 (1998).

 

Selinger, D.A. Lisch, D. and V.L. Chandler. The maize regulatory gene B-Peru contains a DNA rearrangement that specifies tissue-specific expression through both positive and negative promoter elements.  Genetics 149: 1124-1138 (1998).

 

Hollick, J.B. and V.L. Chandler.  Epigenetic allelic states of a maize transcriptional regulatory locus exhibit overdominant gene action.  Genetics 150: 891-897 (1998).

 

Selinger, D.A. and V.L. Chandler.  A mutation in the pale aleurone color1 gene identifies a novel regulator of the maize anthocyanin pathway.  Plant Cell 11: 5-14 (1999).

 

Selinger, D.A. and V.L. Chandler. Major recent and independent changes in levels and patters of expression have occurred at the b gene, a regulatory locus in maize.  Proc. Natl. Acad. Sci. USA 96: 15007-15012 (1999).