GENES ENCODING SMALL GTP-BINDING PROTEINS ANALOGOUS TO MAMMALIAN RAC ARE PREFERENTIALLY EXPRESSED IN DEVELOPING COTTON FIBERS

In animals, the small GTP-binding proteins, Rac and Rho, of the ras su perfamily participate in the signal transduction pathway that regulate s the organization of the actin cytoskeleton. We report here on the ch aracterization of two distinct cDNA clones isolated from a cotton fibe r cDNA library that code for homologs of animal Rac proteins. Using ge ne-specific probes, we have determined that amphidiploid cotton contai ns two genes that code for each of the two Rac proteins, designated Ra c13 and Rac9, respectively. The gene for Rac13 shows highly enhanced e xpression in developing cotton fibers, with maximal expression occurri ng at the time of transition between primary and secondary wall synthe sis. This is also the time at which reorganization of the cytoskeleton occurs, and thus the pattern of expression of Rac13 is consistent wit h its possible role, analogous to animal Rac, in the signal transducti on pathway that controls cytoskeletal organization.