Aaknox1, a kn1-like homeobox gene in Acetabularia acetabulum, undergoes developmentally regulated subcellular localization

Homeobox-containing genes play developmentally important roles in a wide va riety of plants, animals and fungi. As a way of studying how development is controlled in the unicellular green macroalga Acetabularia acetabulum, we used degenerate PCR to clone a knotted1-like (kn1-like) homeobox gene, Aakn ox1 (Acetabularia acetabulum kn1-like homeobox 1). Aaknox1 is the first kno tted1-like homeobox gene to be cloned from a non-vascular plant and shows s trong conservation with kn1-like genes from the vascular plants (ca. 56% am ino acid identity within the homeodomain). Sequencing of cDNA clones indica tes that Aaknox1 possesses at least two distinct polyadenylation sites spac ed ca. 600 bp apart. Southern analysis suggests that several other kn1-like homeobox genes exist in the Acetabularia genome. Northern analyses demonst rate that expression of Aaknox1 is developmentally regulated, with peak lev els of expression during early reproductive phase. Northern analyses furthe r demonstrate that Aaknox1 mRNA undergoes a change in its subcellular local ization pattern during the progression from late vegetative to early reprod uctive phase. In late adult phase, Aaknox1 is distributed uniformly through out the alga; in early reproductive phase, Aaknox1 is present in a gradient with the highest concentration of the mRNA at the base of the stalk, near the single nucleus. These data suggest that Aaknox1 may have a role during early reproductive development and that mRNA localization may be one mechan ism by which A. acetabulum regulates gene expression post-transcriptionally .