Max-Planck-Institut für Züchtungsforschung
Maize pollen expresses a truncated version of the transcriptional coactivator Ada2 --Riehl, M, Dresselhaus, T1, McCormick, S2, Thompson, RD

1University of Hamburg, Applied Plant Molecular Biology II (GERMANY)
2Plant Gene Expression Center, USDA/ARS and UC-Berkeley, Albany, CA (USA)

Transcriptional adaptors and coactivators play important roles in the process of transcription by RNA polymerase II. A genetic screen in yeast led to the identification of the putative adaptors ADA2 (Berger et al., Cell 70(2): 251-65, 1992) which had properties consistent with an adaptor role in transcriptional activation. The Ada2 proteins of different species have been shown to potentiate transcription in vivo and may function as adaptors to bridge physical interactions between DNA-bound activators and the basal transcriptional machinery (Candau et al., J Biol Chem 271(9): 5237-45, 1996). All Ada2 sequences isolated so far share an N-terminal zinc finger motif and a sequence related to the myb DNA-binding domain, as well as a C-terminal Ada3 interaction domain. It has been shown for different species that the N-terminal region of the Ada2 protein is sufficient for an in vitro interaction with Gcn5. Arabidopsis was the first organism for which two Ada2 genes were identified (Stockinger et al., Nucleic Acids Res 29(7): 1524-33, 2001). It was speculated that the two genes may be expressed in different cell types within leaves, roots or other tissues.

We have previously reported on the isolation of central components of the yeast GCN5/ADA2 complex from maize (Becker, MNL 73:22, 1999).

Further analysis of ZmAda2 showed it is present as a small gene family of very closely related sequences (unpublished sequencing results) in the maize genome. Using a probe consisting of the entire cDNA derived from endosperm mRNA on Northern blots, we detected a 2.0 kb mRNA species in all tissues examined except for pollen, which expresses a 1.7 kb version.

Figure 1. Northern blot analysis of ZmAda2. A 1.7 kb mRNA is detected in pollen, whereas in endosperm (and other tissues examined) a 2.0 kb mRNA is detected.

We isolated clones corresponding to the smaller ZmAda2 transcript from a maize pollen cDNA library, and found the pollen transcript (ZmAda2-Pollen) lacks the N-terminal region of the longer, somatic ZmAda2 transcript. The full length ZmAda2 encodes a 565 amino acid long polypeptide, whereas ZmAda2-Pollen, encodes a 387 amino acid long polypeptide, which, apart from being truncated, is identical to the ZmAda2 protein sequence. With an anti-ZmAda2 antibody we are able to detect both predicted proteins in the corresponding maize tissues. ZmAda2-Pollen lacks both the N-terminal zinc finger motif and the myb DNA-binding-like domain. Both full-length and truncated Ada2 proteins were expressed as GST-fusions in E. coli. While full-length ZmAda2 is able to interact with ZmGCN5 in yeast two-hybrid assays, as well as when expressed in E. coli and used in a pull-down assay, ZmAda2-Pollen is not able to interact with ZmGCN5, presumably because of the missing N-terminal domain. Conceivably, ZmAda2-pollen might act as a dominant-negative factor to inhibit targeted histone acetylation by GCN5 or other coactivator activities, in the pollen cells. This might also be related to the production of histone variants in pollen. Further experiments are directed at the possibility that the truncated ZmAda2 protein might influence chromatin organization during pollen maturation.

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