We have isolated a dominant, auxin-insensitive mutant of Arabidopsis thaliana, massugu2 (msg2), that displays neither hypocotyl gravitropism nor phototropism, fails to maintain an apical hook as an etiolated seedling, and is defective in lateral root formation. Yet other aspects of growth and development of msg2 plants are almost normal. These characteristics of msg2 are similar to those of another auxin-insensitive mutant, non-phototropic hypocotyl4 (nph4), which is a loss-of-function mutant of AUXIN RESPONSE FACTOR7 (ARF7) (Harper et al., 2000). Map-based cloning of the MSG2 locus reveals that all four mutant alleles result in amino acid substitutions in the conserved domain II of an Auxin/Indole-3-Acetic Acid protein, IAA19. Interestingly, auxin inducibility of MSG2/IAA19 gene expression is reduced by 65% in nph4/arf7. Moreover, MSG2/IAA19 protein binds to the C-terminal domain of NPH4/ARF7 in a Saccharomyces cerevisiae (yeast) twohybrid assay and to the whole latter protein in vitro by pull-down assay. These results suggest that MSG2/IAA19 and NPH4/ ARF7 may constitute a negative feedback loop to regulate differential growth responses of hypocotyls and lateral root formation.

Cytoplasmic polyadenylation of mRNAs is involved in post-transcriptional regulation of genes, including translational activation. In addition to yeast Cid1 and Cid13 and mouse TPAP, GLD-2 has been recently identified as a cytoplasmic poly(A) polymerase in Caenorhabditis elegans and Xenopus oocytes. In this study, we have characterized mouse GLD-2, mGLD-2, in adult tissues, meiotically maturing oocytes, and NIH3T3 cultured cells. mGLD-2 was ubiquitously present in all tissues and cells tested. mGLD-2 was localized in the nucleus as well as in the cytoplasm of somatic, testicular, and cultured cells. Transfection of expression plasmids encoding mGLD-2 and the mutant proteins into NIH3T3 cells revealed that a 17-residue sequence in the N-terminal region of mGLD-2 probably acts as a localization signal required for the transport into the nucleus. Analysis of reverse transcriptase-polymerase chain reaction indicated the presence of mGLD-2 mRNA in the oocytes throughout meiotic maturation. However, 54-kDa mGLD-2 was found in the oocytes only at the metaphases I and II after germinal vesicle breakdown, presumably due to translational control. When mGLD-2 synthesis was artificially inhibited and enhanced by injection of double-stranded and polyadenylated RNAs into the germinal vesicle-stage oocytes, respectively, oocyte maturation was significantly arrested at the metaphase-I stage. These results suggest that mGLD-2 may act in the ooplasm on the progression of metaphase I to metaphase II during oocyte maturation.

14The elongation of poly(A) tails in cytoplasm is essential for oogenesis and early