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A degeneration of the retinal photoreceptor cells develops in cats when casein is the source of dietary protein. Amino acid profiles indicate that the degeneration is associated with a selective decrease in plasma and retinal taurine concentrations. A sulfur amino acid deficit in the casein diet combined with specific amino acid requirements of the cat appear related to this unique expression of taurine deficiency.

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Production of type IV bundle-forming pili (BFP) by enteropathogenic Escherichia coli (EPEC) requires the protein products of 12 genes of the 14-gene bfp operon. Antisera against each of these proteins were used to demonstrate that in-frame deletion of individual genes within the operon reduces the abundance of other bfp operon-encoded proteins. This result was demonstrated not to be due to downstream polar effects of the mutations but rather was taken as evidence for protein-protein interactions and their role in the stabilization of the BFP assembly complex. These data, combined with the results of cell compartment localization studies, suggest that pilus formation requires the presence of a topographically discrete assembly complex that is composed of BFP proteins in stoichiometric amounts. The assembly complex appears to consist of an inner membrane component containing three processed, pilin-like proteins, BfpI, -J, and -K, that localize with BfpE, -L, and -A (the major pilin subunit); an outer membrane, secretin-like component, BfpB and -G; and a periplasmic component composed of BfpU. Of these, only BfpL consistently localizes with both the inner and outer membranes and thus, together with BfpU, may articulate between the Bfp proteins in the inner membrane and outer membrane compartments. The virulence of enteropathogenic Escherichia coli (EPEC)for orally challenged volunteers (3) requires genes encoded on the 69-kb EPEC adherence factor (EAF) plasmid (31) and within the chromosomal locus of enterocyte effacement (7). The EAF plasmid carries a 14-gene operon that encodes the bundle-forming pilus (BFP), a member of the widely distributed type IV family of pilin proteins (28,29). This operon is required for the production of BFP filaments and virulence; in addition, functional studies of bfp operon mutants show that expression of the operon confers two readily assayable in vitro phenotypes. The localized adherence (LA) phenotype is characterized by circumscribed clusters of bacteria attached to the surface of cell culture monolayers (6, 16). The autoaggregation phenotype (AA) is evident when an overnight culture of dispersed EPEC is inoculated into tissue culture medium; 45 to 60 min later, the bacteria coalesce into dynamic, spherical assemblies which disaggregate after 3 to 4 h, again yielding a suspension of single cells (3).The bfp operon, together with its transcriptional activator BfpTVW/PerABC, which is located elsewhere on the EAF plasmid, (9, 32), is sufficient for expression of BFP filaments and the LA and autoaggregation phenotypes when it is harbored in E. coli strains that normally do not express type IV pili. Thus, the operon's 14 genes appear to encode the minimal set of functions, exclusive of transcription factors and the periplasmic protein, DsbA (34), that are specifically required for BFP biogenesis and function.Details of the environmentally responsive transcriptional regulation of the bfp operon are emerging (21); however, the interactions among the proteins expressed by the operon rema...