Initiator protein dependent binding of messenger RNA to ribosomes

Involvement of ribosomal protein S1 in the assembly of the initiation complex

Antibodies against ribosomal protein S1 (anti-S1) have been used to determine the function of S1 in the partial reactions involved in the translation of MS2 RNA in vitro. Vacant ribosomes are fully sensitive to the antibodies, whereas elongating ribosomes are resistant. We have determined at which stage of translation the resistance to anti-S1 is acquired. We find that insensitivity to anti-S1 already arises upon mixing 30-S subunits with MS2 RNA. Apparently the two particles form a complex in which S1 is functionally protected against its antibody. Complex formation depends on elevated temperature, a suitable ionic environment and it is stimulated by the initiation factor IF-3. It does not depend on IF-1, IF-2 or fMet-tRNA. Thus ribosomes have the potential to recognize the messenger in the absence of fMet-tRNA. Protein S1 appears directly involved in this primary recognition reaction.

Sequence of events in initiation of translation: a role for initiator transfer RNA in the recognition of messenger RNA

It is shown that initiation of translation involves several steps. (i) Binding of fMet-tRNA(fMet) to the bacterial 30S ribosomal subunit in the absence of messenger RNA, yielding a 34S complex. This binding is rapid and dependent on initiation factor 2 but not on initiation factor 3. (ii) Binding of messenger RNA to the 34S complex. This binding is slower and depends on initiation factor 3. If R17 RNA is used as messenger, the resulting complex sediments at 46 S. (iii) Joining of a 50S subunit to yield a complete initiation complex. Binding of fMet-tRNA(fMet) not only precedes, but is necessary for, correct binding of messenger RNA to ribosomes. Thus, initiator tRNA may play an active role in the selection of initiation sites in messenger RNA.

Initial dipeptide formation in hemoglobin biosynthesis

Initiation factors M(1) + M(2) from reticulocyte ribosomes bind Met-tRNA(F) to rabbit reticulocyte ribosomes containing endogenous hemoglobin mRNA. The initial binding of Met-tRNA(F) appears to be to the small ribosomal subunit. The Met-tRNA(F) is able to participate in what is presumed to be the first peptide bond in the formation of hemoglobin, namely the synthesis of a methionyl-valine dipeptide. The formation of this methionyl-valine dipeptide requires Met-tRNA(F), initiation factors M(1), M(2), and M(3), as well as Val-tRNA and T(1). No synthesis of methionyl-valine dipeptide takes place if Met-tRNA(F) is replaced by Met-tRNA(M), or if initiation factor M(3) is omitted. Thus, Met-tRNA(F) appears to be the initiator tRNA for hemoglobin biosynthesis and M(3), although required for the synthesis of the first peptide bond of hemoglobin, does not appear to be necessary, under the experimental conditions studied, for Met-tRNA(F) binding.

Isolation and properties of polypeptide chain initiation factor FII from Escherichia coli: evidence for a dual function

Initiation factor FII (F(3), factor B) has been purified from Escherichia coli Q13 to apparent homogeneity. It is a heat-stable basic protein, consisting of a single polypeptide chain of molecular weight 21,000. FII is required, in addition to the other factors FI and FIII, for the formation of a 70S complex containing fMet-tRNA, poly(U,G) (used as a model messenger RNA), and the ribosome. FII also is capable of dissociating 70S ribosomes to 30S and 50S subunits. Evidence is presented that both activities are mediated by the same protein.