Catalytic diversity of extended hammerhead ribozymes.

Chimeras of the well-characterized minimal hammerhead 16 and nine extended hammerheads derived from natural viroids and satellite RNAs were constructed with the goal of assessing whether their very different peripheral tertiary interactions modulate their catalytic properties. For each chimera, three different assays were used to determine the rate of cleavage and the fraction of full-length hammerhead at equilibrium and thereby deduce the elemental cleavage ( k 2) and ligation ( k -2) rate constants. The nine chimeras were all more active than minimal hammerheads and exhibited a very broad range of catalytic properties, with values of k 2 varying by 750-fold and k -2 by 100-fold.

Heterodimeric GTPase core of the SRP targeting complex.

Two structurally homologous guanosine triphosphatase (GTPase) domains interact directly during signal recognition particle (SRP)-mediated cotranslational targeting of proteins to the membrane. The 2.05 angstrom structure of a complex of the NG GTPase domains of Ffh and FtsY reveals a remarkably symmetric heterodimer sequestering a composite active site that contains two bound nucleotides.

Crystallization of the GMPPCP complex of the NG domains of Thermus aquaticus Ffh and FtsY.

The GTPases Ffh and FtsY are components of the prokaryotic signal recognition particle protein-targeting pathway. The two proteins interact in a GTP-dependent manner, forming a complex that can be stabilized by use of the non-hydrolyzable GTP analog GMPPCP. Crystals of the complex of the NG GTPase domains of the two proteins have been obtained from ammonium sulfate solutions.

Conformational change of the N-domain on formation of the complex between the GTPase domains of Thermus aquaticus Ffh and FtsY.

The structural basis for the GTP-dependent co-translational targeting complex between the signal recognition particle (SRP) and its receptor is unknown. The complex has been shown to have unusual kinetics of formation, and association in vivo is likely to be dependent on catalysis by the SRP RNA. We have determined conditions for RNA-independent association of the 'NG' GTPase domains of the prokaryotic homologs of the SRP components, Ffh and FtsY, from Thermus aquaticus.