DNA replication in cycling eukaryotic cells necessitates the reestablishment of chromatin after nucleosome redistribution from the parental to the two daughter DNA strands. Chromatin assembly factor 1 (CAF-1), a heterotrimeric complex consisting of three subunits (p150/p60/p48), is one of the replication-coupled assembly factors involved in the reconstitution of S-phase chromatin. CAF-1 is required in vitro for nucleosome assembly onto newly replicated chromatin in human cells and Arabidopsis thaliana, and defects in yeast (Saccharomyces cerevisiae) affect DNA damage repair processes, predominantly those involved in genome stability. However, in vivo chromatin defects of caf-1 mutants in higher eukaryotes are poorly characterized. Here, we show that fasciata1-4 (fas1-4), a new allele of the Arabidopsis fas1 mutant defective in the p150 subunit of CAF-1, has a severe developmental phenotype, reduced heterochromatin content, and a more open conformation of euchromatin. Most importantly, homologous recombination (HR), a process involved in maintaining genome stability, is increased dramatically in fas1-4, as indicated by a 96-fold stimulation of intrachromosomal HR. Together with the open conformation of chromatin and the nearly normal expression levels of HR genes in the mutant, this result suggests that chromatin is a major factor restricting HR in plants.

RAD51, the eukaryotic homolog of the bacterial RecA recombinase, plays a central role in homologous recombination (HR) in yeast and animals. Loss of RAD51 function causes lethality in vertebrates but not in other animals or in the flowering plant Arabidopsis thaliana, suggesting that RAD51 is vital for highly developed organisms but not for others. Here, we found that loss of RAD51 function in the moss Physcomitrella patens, a plant of less complexity, caused a significant vegetative phenotype, indicating an important function for RAD51 in this organism. Moreover, loss of RAD51 caused marked hypersensitivity to the double-strand break-inducing agent bleomycin in P. patens but not in Arabidopsis. Therefore, HR is used for somatic DNA damage repair in P. patens but not in Arabidopsis. These data imply fundamental differences in the use of recombination pathways between plants. Moreover, these data demonstrate that the importance of RAD51 for viability is independent of taxonomic position or complexity of an organism. The involvement of HR in DNA damage repair in the slowly evolving species P. patens but not in fast-evolving Arabidopsis suggests that the choice of the recombination pathway is related to the speed of evolution in plants.

TheCONSTANS (CO)gene plays a central role in the regulation of flowering time inArabidopsis,and is a member of a family of 17CO‐likegenes.COandCO‐likegenes have been found in all flowering plants, but not in yeast and animals. To address the question of the origin ofCO,we analysed this gene family in the mossPhyscomitrella patens,a phylogenetically distant organism. Database searches in EST libraries that almost completely covered thePhyscomitrellatranscriptome, and Southern blotting, identified only three genes that had all of the hallmarks ofCO.Further analysis demonstrated that these are most similar toCO‐likegenesAtCOL3/AtCOL4/AtCOL5,a group ofArabidopsisgenes closely related to, but distinct fromCO,suggesting that theCObranch of theAtCOLphylogeny does not exist in thePhyscomitrellagenome. Since 17COLgenes occur inArabidopsisand only three closely related and two distantly related genes were found inPhyscomitrella,the family of CO‐like proteins appears to be smaller inPhyscomitrellathan inArabidopsis,in agreement with observations made with other gene families. The data also indicate thatCO‐likegenes must have existed in the common ancestor of bryophytes and flowering plants, and thatCOoriginated in the group ofCO‐likegenes represented byAtCOL3/AtCOL4/AtCOL5.Furthermore, expression of the three closely relatedPhyscomitrellahomologues is regulated by light, suggesting that the role of CO in flowering time control was probably derived from an ancestral function in light signal transduction.