Histone chaperones AtChz1A and AtChz1B are required for H2A.Z deposition and interact with the SWR1 chromatin-remodeling complex in Arabidopsis thaliana.

The histone variant H2A.Z plays key functions in transcription and genome stability in all eukaryotes ranging from yeast to human, but the molecular mechanisms by which H2A.Z is incorporated into chromatin remain largely obscure. Here, we characterized the two homologs of yeast Chaperone for H2A.Z-H2B (Chz1) in Arabidopsis thaliana, AtChz1A and AtChz1B. AtChz1A/AtChz1B were verified to bind to H2A.Z-H2B and facilitate nucleosome assembly in vitro. Simultaneous knockdown of AtChz1A and AtChz1B, which exhibit redundant functions, led to a genome-wide reduction in H2A.Z and phenotypes similar to those of the H2A.Z-deficient mutant hta9-1hta11-2, including early flowering and abnormal flower morphologies. Interestingly, AtChz1A was found to physically interact with ACTIN-RELATED PROTEIN 6 (ARP6), an evolutionarily conserved subunit of the SWR1 chromatin-remodeling complex. Genetic interaction analyses showed that atchz1a-1atchz1b-1 was hypostatic to arp6-1. Consistently, genome-wide profiling analyses revealed partially overlapping genes and fewer misregulated genes and H2A.Z-reduced chromatin regions in atchz1a-1atchz1b-1 compared with arp6-1. Together, our results demonstrate that AtChz1A and AtChz1B act as histone chaperones to assist the deposition of H2A.Z into chromatin via interacting with SWR1, thereby playing critical roles in the transcription of genes involved in flowering and many other processes.