Polypeptide chain initiation general

We have already discussed primary structure in terms of the general character of amino acids and some specific examples of amino acid sequences in certain proteins will be discussed later. Our attention now is focused on secondary structure, or conformation as we called it when we discussed synthetic polymers. There are a number of factors that afreet the conformation of a polypeptide chain and a lot can be learned initially by just focusing on two of these steric restrictions on bond rotations and the strong driving force for amide groups to hydrogen bond to one another. 

By contrast, eucaryal mRNAs are translated after extensive modifications of the primary transcripts that yield mature (generally capped and polyadenylated) monocistronic mRNAs. Recognition of translation start sites does not rely upon Shine-Dalgarno recognition instead, the small ribosomal subunit (generally) binds to the capped 5 end of mRNA and scans its nucleotide sequence until the initiator AUG codon is encountered. The polypeptide chains are initiated by a non-formylated methionine and the initiation reactions are aided by as many as 8-10 protein factors, some of which possess ATPase activity and perform functions not encountered in bacteria, such as cap recognition and mRNA unwinding (for a detailed review see ref [4]). 

The departure of the Perrin equation from linearity may be considered more generally. A curvature convex to the T/rj axis may result from the complete kinetic dissociation of rigid units or from increased rotational freedom within the polypeptide chain 26, 32, 65, 66, 68). A curvature concave to the T/rj axis may occur whenever more than one rotational relaxation time is present and may be detected when the two rotational relaxation times differ by more than fivefold 65). The initial slope, for small values of T/tj, depends upon the harmonic mean of the relaxation times involved and is determined practically by the shorter one. Such curvature has been reported in studies with DNS-labeled polyvinylamine and poly-p-amino-styrene 22, 33, 64). By applying a theory which deals with the case of an emitting group which has only one degree of rotational freedom, Gottlieb and Wahl 33) concluded that the rotations activated by temperature in... 

Cells incubated in medium without one or all of the essential amino acids also show a decline in protein synthesis over the first hour, involving an inhibition of both polypeptide chain elongation and initiation (Van Venrooij et al., 1972 Vaughan et al, 1971). Inhibition is fully reversible on readdition of the amino acid(s), even in the presence of actinomycin D, and, again, initiation appears to be the more sensitive of the two steps. It is not known whether this effect is a result of some general control of the rate of initiation by the intracellular level of all 20 amino acids or their charged tRNA s, or whether it is an indirect result of a decrease in the level of ATP (Van Venrooij et al, 1972). 

It makes good sense that initiation is the rate-limiting step in mRNA translation. Once synthesis of a protein chain has been initiated, the remainder of its synthesis follows more or less automatically. The crucial step for control, then, is that involving the attachment of an mRNA molecule to a 40 S ribosomal subunit. Indeed, most cases of translational control concern this step. In general, once mRNA has entered a 40 S initiation complex, formation of the complete encoded polypeptide chain is virtually assured. Hence, this review will concentrate on the properties of mRNA and the translation components involved in the events leading up to this complex. 

---