(1) Nucleotide analogs have been introduced as structural probes into active sites of dodecameric glutamine synthetase from Escherichia coli. These enzyme derivatives are being used for X- ray structural analysis in D. Eisenberg's laboratory at UCLA and for determining relative intra-subunit distances between the active sites and Trp 57 and Trp 158. (2) Reversible temperature-induced transitions of glutamine synthetase involve the exposure of 1 of the 2 Trp residues/subunit and 2 of the 17 Tyr residues/subunit without dissociation or aggregation of the dodecameric enzyme. Thermally induced transitions appear to involve the melting of active site structures and at least 2 domains unfold independently. (3) Thermodynamic studies of Zn(II)-induced stacking of glutamine synthetase dodecamers are in progress. (4) Regulatory subunits of E. coli aspartate transcarbamoylase bind Zn(lI) with high affinity; kinetic and equilibrium constants for this interaction have been determined. (5) The structure and function of transcriptional factor TFIIIA from oocytes of Xenopus laevis are being investigated with respect to its proposed """"""""Zn(II) finger"""""""" structure and specific interactions between TFIIIA and 5S RNA, DNA encoding for 5S RNA, and RNA polymerase III, and possibly other transcriptional protein factors.
