DEVELOPMENTAL AND CELL-CYCLE REGULATION OF ALFALFA NUCMS1, A PLANT HOMOLOG OF THE YEAST NSR1 AND MAMMALIAN NUCLEOLIN

We report here the isolation and characterization of the nucMs1 alfalf a cDNA, whose predicted amino acid sequence structurally resembles the yeast Nsr1 protein and animal nucleolins. These proteins consist of a n N-terminal acidic domain, centrally located RNA recognition motifs ( RRMs), and a C-terminal glycine- and arginine-rich domain. In comparis on with animal nucleolins that contain four RRMs, NucMs1 more closely resembles the yeast Nsr1 protein, which contains only two RRMs. A NucM s1 C-terminal peptide antibody specifically recognized a 95-kD nucleol ar protein in alfalfa cells that changed its localization in a cell cy cle-dependent manner. The nucMs1 transcript and p95(nucMs1) protein le vels correlated with cell proliferation, and nucMs1 gene expression wa s found to be induced in the G(1) phase upon mitogenic stimulation of G(0)-arrested leaf cells. In situ hybridization analysis of different alfalfa organs during various developmental stages showed that nucMs1 gene expression is highest in root meristematic cells, but it is also found in other meristematic cells of the plant body, nucMs1 expression is tightly linked to cell proliferation but does not depend on a part icular cell cycle phase. No nucMs1 expression was observed in cells th at had exited the cell cycle and were undergoing differentiation or po lar growth, indicating that nucMs1 may not be necessary for processes other than cell proliferation.