The regulation of sporulation-specific transcription in Phytophthora
infestans
Qijun Xiang, Audrey Ah Fong and Howard Judelson
The asexual sporangia produced by P. infestans play an important role in spreading disease.
Microarray analysis identified a number of genes specifically expressed during sporulation. This
study focuses on identifying the transcription factor binding sites in the promoters of those genes,
and the matching transcription factors. The promoters of two genes (Cdc14 and Pks1) have been
characterized in detail. The Cdc14 promoter carries a motif with a cluster of three repeats having
the consensus CT(C/T)AAC. This motif determines not only the strength of transcription, but also
the specificity of the site of transcription initiation. The motif is bound by a protein from nuclear
extracts in electrophoretic mobility shift assays (EMSA), and the binding requires at least two
adjacent repeats. By following the DNA binding activity, we are purifying the binding protein with
conventional chromatography. The other promoter (Pks1) carries a regulatory region with low
nucleosome-binding potential. This region contains at least four sites that function by activating or
suppressing transcription. One of the activating regions contains two adjacent protein binding
sites. One of the binding sites is comprised of two tandem repeats, which functions to activate
transcription and which is significantly enriched in the promoters of the sporulation-specific genes.
The binding activity to this motif is sporangia-specific. The other protein-binding site has a
consensus similar to eukaryotic Inr, where the general transcription machinery binds to initiate
transcription, but it has significantly higher protein-binding affinity than a generic Inr. This
consensus is under-represented in the genome, suggesting that it has been selected against
during genome evolution. In conclusion, our study indicates that sporulation-specific genes
possess promoter structures with diversified regulatory modules. This may reflect the diverse
activities of the transcriptome during sporulation, which are needed to regulate sporangia
development, prepare the sporangia for rapid differentiation into zoospores or infection stuctures,
or maintain the metabolic state of the sporangium.
Department of Plant Pathology and Microbiology, University of California, Riverside, CA 92521 USA