To examine the influence of bases contiguous to a starter codon, a rapid means of assembling biologically active regions corresponding to portions, or to analogues of portions of intercistronic regions is desirable. To do this, a chemical method for the specific insertion of 3′-monophosphate groups on to chemically synthesized trinucleotides, tetranucleotides and hexanucleotides of defined sequence bearing different bases at the 3′ terminus has been devised. The method involved phos- phorylation of blocked oligoribonucleotides synthesized by a phosphotriester method. The deblocked oligoribonucleotides were phosphorylated at the 5′ end with T4-induced polynucleotide kinase. The products of this kinase reaction served as 'donors' in RNA ligase reactions. The [32P]pC-A-U-A-U-Gp, [32P]pA-U-Gp, [32P]pU-A-A, [32P]pA-G-G-Ap, [32P]pC-U-U-Ap and [32P]pU-C-C-Up 'donors' were used to synthesize A-G-G-A[32P]pC-A-U-A-U-Gp, U-C-C-U[32P]pC-A-U-A-U-Gp, U-A- A[32P]pA-U-G, A-U-G[32P]pU-A-A, U-A-A-G[32P]pA-G-G-Ap, U-C-C-U[32P]pC-U-U-Ap and A-U-U-C[32P]pU-C-C-Up indicating that the method functions with all bases. A-U-Gp, pA-U-Gp and pC-A-U-A-U-Gp were isolated free of reactants and, along with pA-U-G, were all shown to promote the formation of translational initiation complexes. A-G-G-A[32P]pC-A-U-A-U-Gp, which corresponds to the 5′-terminal portion of the intracistronic region of the maturation protein of bacteriophage Qβ, bound more efficiently to Escherichia coli ribosomes than the U-C-C-U[32P]pC- A-U-A-U-Gp, [32P]pC-A-U-A-U-Gp, [32P]PA-U-G-U-A-A or [32P]pU-A-A-A-U-G controls. The results suggested that the content and number of residues at the 5′ terminus attached to A-U-G affect binding of oligonucleotides to ribosomes; purine nucleosides appear to be more effective than py- rimidine nucleosides in this regard. [ABSTRACT FROM AUTHOR]