Effect of changes in the flexible arm on tRNase Z processing kinetics

Area of Science:

• Molecular Biology
• Enzymology
• Biochemistry

Background:

• Transfer RNAs (tRNAs) undergo crucial processing for aminoacylation and translation.
• tRNase Z enzyme is vital for tRNA maturation, specifically removing the 3' end trailer.
• A flexible arm (FA) on tRNase Z, featuring an alpha/beta hand, binds pre-tRNA elbows.

Purpose of the Study:

• To investigate the role of the flexible arm (FA) in tRNase Z substrate binding and catalysis.
• To identify specific residues within the FA that are critical for enzyme-substrate interactions.
• To elucidate the mechanism of tRNA precursor processing by tRNase Z.

Main Methods:

• Site-directed mutagenesis of the tRNase Z flexible arm.
• Enzyme kinetics assays (measuring Km and kcat) to assess substrate binding and catalytic efficiency.
• Alanine scanning mutagenesis of the FA region.
• Analysis of co-crystal structures to predict substrate contact points.

Main Results:

• Deletion of the FA hand significantly increased the Km (substrate binding affinity) by ~100-fold, with minimal impact on kcat (catalytic rate).
• A conserved leucine residue within the FA hand was identified as critical for substrate binding, nearly eliminating binding upon substitution.
• Mutations in the GP loop and other predicted substrate-contacting residues also increased Km.
• Substitutions in the beta10-beta11 loop primarily affected kcat, indicating roles in catalysis.

Conclusions:

• The flexible arm (FA) of tRNase Z is essential for efficient substrate recognition and binding.
• Specific residues, including a conserved leucine, play a critical role in mediating the interaction between tRNase Z and pre-tRNA.
• The FA hand contributes significantly to the enzyme's catalytic efficiency by ensuring proper substrate positioning.