Cellulose, an abundant, crystalline polysaccharide, is central to plant morphogenesis and to many industries. Chemical and ultrastructural analyses together with map-based cloning indicate that the RSW1 locus of Arabidopsis encodes the catalytic subunit of cellulose synthase. The cloned gene complements the rsw1 mutant whose temperature-sensitive allele is changed in one amino acid. The mutant allele causes a specific reduction in cellulose synthesis, accumulation of noncrystalline beta-1,4-glucan, disassembly of cellulose synthase, and widespread morphological abnormalities. Microfibril crystallization may require proper assembly of the RSW1 gene product into synthase complexes whereas glucan biosynthesis per se does not.

Escherichia coli K-12 has long been known not to produce an 0 antigen. We recently identified two independent mutations in different lineages of K-12 which had led to loss of 0 antigen synthesis (D. Liu and P. R. Reeves, Microbiology 140:49-57, 1994) and constructed a strain with all rjb (0 antigen) genes intact which synthesized a variant of 0 antigen 016, giving cross-reaction with anti-017 antibody. We determined the structure of this 0 antigen to be -with an 0-acetyl group on C-2 of the rhamnose and a side chain c-D-Glcp on C-6 of GlcNAc. 0 antigen synthesis is rfe dependent, and D-GlcpNAc is the first sugar of the biological repeat unit. We sequenced the rjb (0 antigen) gene cluster and found 11 open reading frames. Four rhamnose pathway genes are identified by similarity to those of other strains, the rhamnose transferase gene is identified by assay of its product, and the identities of other genes are predicted with various degrees of confidence. We interpret earlier observations on interaction between the rjb region ofEscherichia coli K-12 and those ofE. colh 04 and E. coi Flexneri. All K-12 rjb genes were of low G+C content for E. coil. The rhamnose pathway genes were similar in sequence to those of (Shigella) Dysenteriae 1 and Flexneri, but the other genes showed distant or no similarity. We suggest that the K-12 gene cluster is a member of a family of rjb gene clusters, including those of Dysenteriae 1 and Flexneri, which evolved outside E. coli and was acquired by lateral gene transfer.Escherichia coli K-12 was isolated in 1922 and used as a standard E. coli strain at Stanford University for many years; the strains which survive all derive from cultures given to E. Tatum and others in the 1940s and early 1950s, when E. coli K-12 was first used for the genetic studies which led to its adoption as the major strain for laboratory study. After 50 years of intensive study, E. coli K-12 is arguably the best understood of all organisms, having been used for studies of many facets of living organisms, outlined in a two-volume book on E. coli and Salmonella enterica (45). Currently, 50% of its genome has been sequenced (56), and completion of this task will increase the focus of attention on E. coli K-12 for studies which integrate genomic information and biochemical processes.There are, however, significant gaps in our knowledge of E. coli K-12. During its first 25 to 30 years in the laboratory, it probably accumulated a range of mutations which improved adaptation to a laboratory environment but destroyed its ability to survive in its natural environment. Among them were mutations in the rjb gene cluster which led to loss of 0 antigen synthesis. The 0 antigen, a repeat unit polysaccharide which is a component of lipopolysaccharide (LPS), is the major surface antigen of many gram-negative bacteria (see reference 53 for a review) and, for E. coli, was present in by far the majority of strains when first isolated. However, it is often lost during culture, presumably because it offers no advantage under suc...

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