SUMMARY The Fe protein of the nitrogenase of the unicellular Cyanobacterium Gloeothece (Nägeli) sp. ATCC 27152 can be resolved by SDS–PAGE into two antigenically detectable components of approximate Mr 38500 and 40000 respectively. The larger form of this protein may be produced by modification of the smaller form. Modification of the Fe protein is promoted under conditions where O2 (but not O2− or H2O2) has increased access to the enzyme, but does not allow nitrogenase to function under conditions of O2 stress. During growth of Gloeothece under alternating light and darkness, antigenically detectable Fe protein is absent throughout most of the light period. The restriction of nitrogenase activity to the period of darkness is better explained in terms of regulation of nitrogenase synthesis and degradation than by reversible modification of a constant intracellular concentration of Fe protein. However, newly synthesized Fe protein always appeared initially as its larger form, which may be catalytically inactive in aerobic cultures of Gloeothece. Conversion of this form to the smaller, assumed active, form of the Fe protein may be an additional factor in explaining the increase in nitrogenase activity that occurs during the first few hours of each dark phase.

No abstract

Cell‐permeable peptide import recently was developed to deliver synthetic peptides into living cells for studying intracellular protein functions. This import process is mediated by an N‐terminal carrier sequence which is the hydrophobic region of a signal peptide. In this study, the conformational consequence of the interaction of cell‐permeable peptides with different mimetic membrane environments was investigated by circular dichroism analysis. We showed that cell‐permeable peptides adopted a‐helical structures in sodium dodecyl sulfate (SDS) micelles or aqueous trifluoroethanol (TFE). The potency of these peptides in forming helical structures is higher in an amphiphilic environment (SDS) than in a hydrophobic environment (TFE), suggesting that some hydrophilic molecules associated with the cell membrane may be involved in peptide import. We also studied topological requirements of cell‐permeable peptide function. We demonstrated that peptides containing the carrier sequence in their C‐termini can also be imported into cells efficiently. This important discovery can avoid repetitious synthesis of the membrane‐translocating sequence for peptides with different functional cargoes and is potentially useful for developing a cell‐permeable peptide library. Finally, we showed that, when a retro version of the carrier sequence was used, the peptide lost its translocating ability despite retaining a high content of a‐helical structure in mimetic membrane environments. This suggests that the propensity of peptides to adopt a helical conformation is required but not sufficient for cellular import and that other structural factors such as the side‐chain topology of the carrier sequence are also important. Our studies together contribute to the more rational design of useful cell‐permeable peptides. © Munksgaard 1998.