Analysis of Saccharomyces cerevisiae histone H3 mutants reveals the role of the alphaN helix in nucleosome function.

To understand the role of histone H3 sub-domains in chromatin function, 35 histone H3 tandem alanine mutants were generated and tested for their viability and sensitivity to DNA damaging agents. Among 13 non-viable H3 mutants, 6 were mapped around the alphaN helix and preceding tail region. Mutants with individual alanine substitutions in this region were viable but developed multiple sensitivities to DNA damaging agents. The only viable triple mutant, REI49-50A, in the alphaN helix region could not grow when combined with histone chaperone mutations, such as asf1Delta, cac1Delta, or hir1Delta, suggesting that this particular region is important when the histone assembly/disassembly pathway is compromised. In addition, further analysis showed that T45, E50, or F54 of the alphaN helix genetically interacted with a histone chaperone (Asf1) and transcription elongation factors (Paf1 and Hpr1). These results suggest a specific role of the H3 alphaN helix in histone dynamics mediated by histone chaperones, which might be important during transcription elongation.