Cotton, an important natural fiber, is a differentiated epidermal cell. The number of genes that are active in fiber cells is similar to those in leaf, ovule, or root tissues. Through differential screening of a fiber cDNA library, we isolated five cDNA clones that are preferentially expressed in fiber. One of the cDNA clones, pCKE6, corresponded to an abundant mRNA in fiber. Transcripts for E6 were detected throughout the development of the fiber. Immunoprecipitation of in vitro translation products and Western blot analysis of fiber proteins showed two polypeptides in the range of 30-32 kDa as the products of E6 mRNA. Sequence analysis and hybrid-selected RNA translation also suggest that E6 mRNAs encode two polypeptides. Concentrations of E6 mRNA and protein are highest during the late primary cell wall and early secondary cell wall synthesis stages. Sequence comparison of E6 with other known eukaryotic and prokaryotic genes reveals no significant homology (GenBank; December 1991). E6 or a homologous gene(s) is conserved in several members of Malvaceae as well as in one other fiber-producing plant, kapok, but is not found in several other plants examined or in Acetobacter xylinum. A genomic clone corresponding to pCKE6 was isolated, and the promoter element of the E6 gene was shown to direct the expression of a carrot extensin mRNA in a tissuespecific and developmentally regulated fashion in transgenic cotton plants.Cotton is the most important textile fiber crop; therefore, the development of cotton fiber and the biosynthesis of cellulose are significant areas of research. Cotton fiber is a differentiated single epidermal cell of the outer integument of the ovule. It has four distinct growth phases; initiation, elongation (primary cell wall synthesis), secondary cell wall synthesis, and maturation (for a review, see ref. 1). Initiation of fiber development appears to be triggered by hormones, auxins, and gibberellins (for a review, see ref.2). The primary cell wall is laid down during the elongation phase, lasting up to 25 days postanthesis (DPA). Synthesis of the secondary wall commences prior to the cessation of the elongation phase and continues to -40 DPA, forming a wall (5-10 ,um) ofalmost pure cellulose. Upon maturity, cotton fiber is -87% cellulose. Thus cell wall biosynthesis is a major synthetic activity in fiber cells. The cell wall components are synthesized and transported by a functionally integrated membrane system of endoplasmic reticulum, Golgi complex, and plasmalemma. Newly synthesized cell wall polypeptides are released into the endoplasmic reticulum lumen before transportation and incorporation into cell walls. Glycosylation of structural proteins, as well as polymerization of hemicellulose and pectin, takes place in the Golgi complex from which the products are released into plasmalemma through the fusion of vesicles (1, 3).Plants, as well as certain bacteria, synthesize cellulose through enzyme systems localized in plasma membranes that use UDP-glucose as a substrate (1). The en...