r-selected species

High rate of per capita population growth

poor  competitive ability

Grows quickly 

age early, 

devoting much energy to producing  large number of progeny 

Small in size, and individuals do not live long. 

Random matting

TLS Online TPP Program

#Id: 7071

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 7072

#Unit 3. Fundamental Processes

The charging of a tRNA with its cognate amino acid is a remarkably accurate process.

Experimental measurements indicate, for example, that at equal concentrations of isoleucine and valine, IleRS transfers ∼40,000 isoleucines to tRNAIle for every valine it so transfers.

TLS Online TPP Program

#Id: 7073

#Unit 3. Fundamental Processes

This rejection of noncognate tRNAs at a stage of the reaction that precedes the synthesis of misacylated tRNA is sometimes referred to as kinetic proofreading.

TLS Online TPP Program

#Id: 7074

#Unit 3. Fundamental Processes

 Valyl-tRNA synthetase can sterically exclude isoleucine from its catalytic pocket because isoleucine is larger than valine. In contrast, valine should slip easily into the catalytic pocket of the isoleucyl-tRNA synthetase. Although both amino acids will fit into the isoleucyl-tRNA synthetase amino acid–binding site, interactions with the extra bonding with tyrosine’s OH group. Since no other amino acid resembles tyrosine, the enzyme can do without an editing function

TLS Online TPP Program

#Id: 7075

#Unit 3. Fundamental Processes

isoleucyl-tRNA synthetase has a nearby editing pocket (a deep cleft in the enzyme) that allows it to proofread the product of the adenylylation reaction. AMP-valine (as well as adenylylates of other small amino acids, such as alanine) can fit into this editing pocket, where it is hydrolyzed and released as free valine and AMP. In contrast, AMP-isoleucine is too large to enter the editing pocket and is therefore not subject to hydrolysis. As a consequence, isoleucyl-tRNA synthetase discriminates against valine twice: in the initial binding and adenylylation of the amino acid (discriminating by a factor of 100), and then in the editing of the adenylylated amino acid (again discriminating by a factor of 100), for an

overall selectivity of 10,000-fold (i.e., an error rate of 0.01%).

TLS Online TPP Program

#Id: 7076

#Unit 3. Fundamental Processes