Abstract. We tested the hypothesis that kinesin moves parallel to the microtubule's protofilament axis. We polymerized microtubules with protofilaments that ran either parallel to the microtubule's long axis or that ran along shallow helical paths around the cylindrical surface of the microtubule. When gliding across a kinesin-coated surface, the former microtubules did not rotate. The latter microtubules, those with supertwisted protofilaments, did rotate; the pitch and handedness of the rotation accorded with the supertwist measured by electron cryo-microscopy. The results show that kinesin follows a path parallel to the protofilaments with high fidelity. This implies that the distance between consecutive kinesin-binding sites along the microtubule must be an integral multiple of 4.1 nm, the tubulin monomer spacing along the protofilamerit, or a multiple of 8.2 nm, the dimer spacing.

Vertebrate retinal photoreceptors recover from photoexcitation-induced hydrolysis of guanosine 3', 5'-monophosphate (cyclic GMP) by resynthesizing cyclic GMP, which reopens cation channels that have been closed by light. Activation of guanylate cyclase by light-induced depletion of cytosolic calcium is a key event in this recovery process. This cyclase has now been shown to be regulated by a 23-kilodalton calcium binding protein. The protein is present in both rod and cone photoreceptors and was named recoverin because it promotes recovery of the dark state. The amino acid sequence of recoverin exhibits three potential calcium binding sites (EF hands). That recoverin binds calcium was confirmed with calcium-45 and by observing calcium-induced changes in its tryptophan fluorescence. Recoverin activated guanylate cyclase when free calcium was lowered from 450 to 40 nM, an effect that was blocked by an antibody to recoverin. Thus, guanylate cyclase in retinal rods is stimulated during recovery by the calcium-free form of recoverin. A comparison of recoverin with other calcium binding proteins reveals that it may represent, along with the protein visinin, a family of proteins that are regulated by submicromolar calcium concentrations.

Increasing numbers of Neisseria gonorrhoeae strains with decreased susceptibilities to ceftriaxone and other oral cephalosporins widely used for the treatment of gonorrhea have been isolated in Sydney, Australia, over several years. In this study, we examined the complete penicillin-binding protein 2 (PBP 2) amino acid sequences of 109 gonococci, selected on the basis of their diverse temporal and geographic origins and because they exhibited a range of ceftriaxone MICs: <0.03 g/ml (n ‫؍‬ 59), 0.06 g/ml (n ‫؍‬ 43), and 0.125 g/ml (n ‫؍‬ 7). Auxotyping, serotyping, and genotyping by N. gonorrhoeae multiantigen sequence typing sequence-based analysis was also performed. In total, 20 different amino acid sequence patterns were identified, indicating considerable variation in the PBP 2 sequences in this study sample. Only some of the N. gonorrhoeae isolates with significantly higher ceftriaxone MICs contained a mosaic PBP 2 pattern, while more isolates exhibited a nonmosaic PBP 2 pattern containing an A501V substitution. Although particular N. gonorrhoeae genotypes in our sample were shown to be less susceptible to ceftriaxone, the reduced susceptibility to ceftriaxone was not specific to any particular genotype and was observed in a broad range of auxotypes, serotypes, and genotypes. Overall, the results of our study show that N. gonorrhoeae strains exhibiting reduced sensitivity to ceftriaxone are not of a particular subtype and that a number of different mutations in PBP 2 may contribute to this phenomenon.

Diagnostic, genotypic and antibiotic-resistance determinants of Neisseria gonorrhoeae were analysed by molecular methods to verify the failure of ceftriaxone treatment in two cases of pharyngeal gonorrhoea. Monoplex assays were needed to define competitive inhibition of a positive Chlamydia PCR in a duplex assay. Different penA changes were detected in the N. gonorrhoeae isolated from the two cases. These were associated with raised ceftriaxone MICs of 0.03 and 0.016 mg l−1, which may have contributed to the treatment failures in these cases.