A Novel H2A/H4 Nucleosomal Histone Acetyltransferase in Tetrahymena thermophila

Abstract

Recently, we reported the identification of a 55-kDa polypeptide (p55) from Tetrahymena macronuclei as a catalytic subunit of a transcription-associated histone acetyltransferase (HAT A). Extensive homology between p55 and Gcn5p, a component of the SAGA and ADA transcriptional coactivator complexes in budding yeast, suggests an immediate link between the regulation of chromatin structure and transcriptional output. Here we report the characterization of a second transcription-associated HAT activity from Tetrahymenamacronuclei. This novel activity is distinct from complexes containing p55 and putative ciliate SAGA and ADA components and shares several characteristics with NuA4 (for nucleosomal H2A/H4), a 1.8-MDa, Gcn5p-independent HAT complex recently described in yeast. A key feature of both the NuA4 and Tetrahymena activities is their acetylation site specificity for lysines 5, 8, 12, and 16 of H4 and lysines 5 and 9 of H2A in nucleosomal substrates, patterns that are distinct from those of known Gcn5p family members. Moreover, like NuA4, the Tetrahymena activity is capable of activating transcription from nucleosomal templates in vitro in an acetyl coenzyme A-dependent fashion. Unlike NuA4, however, sucrose gradient analyses of the ciliate enzyme, following sequential denaturation and renaturation, estimate the molecular size of the catalytically active subunit to be ∼80 kDa, consistent with the notion that a single polypeptide or a stable subcomplex is sufficient for this H2A/H4 nucleosomal HAT activity. Together, these data document the importance of this novel HAT activity for transcriptional activation from chromatin templates and suggest that a second catalytic HAT subunit, in addition to p55/Gcn5p, is conserved between yeast and Tetrahymena.

ACKNOWLEDGMENTS

We are grateful to J. Horiuchi and L. Guarente as well as S. Roberts and F. Winston for providing the Ada1p and Ada5p/Spt20p antisera used in this study, respectively, and to Tamara Ranalli for her help in characterizing these antibodies by using Tetrahymena samples. We are also grateful to Stéphane Allard for his help in purification of NuA4 samples.

This work was supported by grants from the NIH (GM53512 to C.D.A.), from HHMI (to J.L.W.), and from the Medical Research Council (MRC) of Canada (to J.C.) J.C. is a Canadian MRC Scholar.