The SAGA continues: expanding the cellular role of a transcriptional co-activator complex

Throughout the last decade, great advances have been made in our understanding of how DNA-templated cellular processes occur in the native chromatin environment. Proteins that regulate transcription, replication, DNA repair, mitosis and other processes must be targeted to specific regions of the genome and granted access to DNA, which is normally tightly packaged in the higher-order chromatin structure of eukaryotic nuclei. Massive multiprotein complexes have been discovered, which facilitate access to DNA and recruitment of downstream effectors through three distinct mechanisms: chemical modification of histone amino-acid residues, ATP-dependent chromatin remodeling and histone exchange. The yeast Spt-Ada-Gcn5-Acetyl transferase (SAGA) transcriptional co-activator complex regulates numerous cellular processes through coordination of multiple histone post-translational modifications. SAGA is known to generate and interact with a number of histone modifications, including acetylation, methylation, ubiquitylation and phosphorylation. Although best characterized for its role in regulating transcriptional activation, SAGA is also required for optimal transcription elongation, mRNA export and perhaps nucleotide excision repair. Here, we discuss findings from recent years that have elucidated the function of this 1.8-MDa complex in multiple cellular processes, and how misregulation of the homologous complexes in humans may ultimately play a role in development of disease.