Preparative RNA-ligase synthesis of decaribonucleotides, the 5'-and 3'-constituent parts to be used for the synthesis of 20-base polyribonucleotides] simulating minimal translation initiation regions for phage RNA was carried out. The decamers were obtained via appropriate heptamers also by RNA-ligase catalyzed synthesis. Apart from decamers used to prepare the functionally active 20-base polyribonucleotide, the minimal translation initiation region of the replicase gene (R) in MS2 and fr phage--sequence R(-17----3) and two its variants, decanucleotides for other template modification were also synthesized. Three 5'-terminal decamers were isolated and identified including the natural decamer ApApApCpApUpGpApGpG (-17----(-)8) and those with G(-9)----A(-9) and U(-12)----C(-12) nucleotide substitutions, as well as three 3'-terminal products differing from the natural region ApUpUpCpCpCpApUpG (-7----3) in MS2 RNA by U(-6)----A(-6), U(-6)U(-5)----A(-6)A(-5) and CCC----UUU (-3----(-)1) substitutions.