Basic research on protein structure, function, and regulation has produced the following results: (1) Heat-induced reversible partial unfolding of dodecameric Mn- glutamine synthetase (Mn-GS) from Escherchia coli at pH 7 has been studied by spectral techniques and by differential scanning calorimetry (DSC). A single endotherm (centered at 51.6 deg C) for two two-state transitions was observed: calorimetric DeltaH = 176 +/- 12 kcal/(mol dodecamer). Cooperative interactions apparently link partial unfolding reactions of all subunits within the GS dodecamer. Variable effects of active-site ligands can be described in thermodynamic terms. (2) Thermal denaturation of the tryptophan synthase complex (+/- bound pyridoxal phosphate) from Salmonella typhimurium also is being studied. DSC profiles for the holoenzyme exhibit two well resolved endotherms corresponding to 2alpha and betabeta domains in the multienzyme complex. Results indicate that: (a) thermally induced unfolding of the alphabetabetaalpha complex is approximately 60% without significant disruption of alpha:beta contacts; (b) the refolding processes are slowly reversible; and (c) a pre-transition exposure of Trp residues in beta chains occurs. (3) Phosphorylated and dephosphorylated myosin-II from Acanthamoeba were found to have identical DSC profiles with a single sharp endotherm at approximately 42 deg C. The unfolding of the rod and head portions of myosin-II appears to be uncoupled in the presence of an ATP analogue. (4) Transcriptional factor IIIA (TFIIIA) from immature oocytes of Xenopus laevis has been specifically labeled at Cys-287 with a highly fluorescent probe (IAEDANS). DNA fragments from the internal control region (ICR) of the 5 S RNA gene also have been labeled at four specific positions with fluorescein and purified. Fluorescence resonance energy transfer measurements on the 1:1 ICR DNA:TFIIIA complex are being made to obtain distances between probes and interaction constants. The wild- type and a mutant form of TFIIIA also have been cloned.
