Question

Compare and contrast how the initiation complex is assembled in bacteria (E. coli) and in eukaryotes (budding yeast cells). How do the bacterial and budding yeast ribosomes determine which start codon to initiate translation? Include in your description a discussion of the cis-acting elements, trans-acting factors, and energy requirements required for building the initiation complex in each system. The best response for this question will NOT BE A LIST of the requirements. It will clearly COMPARE and CONTRAST this process in bacteria and budding yeast.

Answer 1

In term of initiator-

In eukaryotes,the initaiting amino acid is methionine rather than N- formylmethionine.However, as in prokaryotes,a special tRNA participates in initiation.This aminoacyl- tRNA is called met-tRNA(i) or met- tRNA(f) ,where i and f stand for initiation and formylated in vitro.

Ribosomes: Eukaryotic ribosomes are larger .They consist of a 60S large subunit and a 40S small subunit, Which come together to form an 80S particle having a mass of 4200 kd, compared with 2700 kd for the prokaryotic 70S ribosome. The 60S subunit contains three RNAs: the 5s and 28s RNA are the counterparts of the prokaryotic 5s and 23S molecules;its 5.8S RNA is unique to eukaryotes.

Initiation: The initiating codon in eukaryotes id always AUG. Eukaryotes,in contrast with prokaryotes,do not use a specific purine -rich sequence on the 5' side to distinguish initiator AUGs from internal ones.Instead,the AUG nearest the 5' end of mRNA is usually selected as the start site. A 40S ribosome attacehs to the cap at the 5' end of eukaryotic mRNA and searches for AUG codon by moving step-by step in the 3' direction.In almost all caes,eukaryotic mRNA has only one start site and hence is the template for a single protein.In contrast, a prokaryotic mRNA can have multiple shine- Dalgarno sequence and, hence,start sites,and it can serve as a template for factors than do prokaryotes, and their interplay is much more intricate.The difference in initiation mechanism between prokaryoties and eukaryotes is ,in part, a consequence of the difference in RNA processing.The 5' end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre - mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initiated.Thus, there is ample opportunity for the formation of complex secondary structures that must be removed to expose signals in the mature mRNA. The 5' cap provide an easily recognizable starting point.In addition, the complexity of eukaryotic translation initiation provides another mecahnism for gene expression.