BASE-PAIRING OF 23 S RIBOSOMAL-RNA ENDS IS ESSENTIAL FOR RIBOSOMAL LARGE SUBUNIT ASSEMBLY

In ribosomal RNA precursors the spacer sequences bracketing mature 16 S and 23 S rRNA are base-paired to form long helices (processing stems ). In pre-23 S rRNA, the processing stem is continued by eight basepai rs of mature 23 S rRNA known as helix 1. Recently, we have found that any part of 23 S rRNA between positions 40 and 2773 could be deleted w ithout the loss of ribosome-like particle formation, while both end re gions were indispensable. In this paper we have analyzed the role of t he 5' and 3' end regions of 23 S rRNA during ribosomal 50 S assembly i n vivo by using mutants of the 23 S rRNA gene. Deletions and substitut ions in both strands of the helix 1 lead to the loss of plasmid derive d 50 S formation. Compensatory mutations restoring helix 1 were assemb led into functional 50 S subunits. We conclude that the helix 1 of 23 S rRNA is the main RNA determinant for ribosomal large-subunit assembl y. Deletions in both the 5' and 3' strand of the processing stem reduc ed the ability of the 23 S rRNA to form ribosomal 50 S subunits. Howev er, even the complete removal of either the 5' or the 3' strand of the processing stem did not abolish the 50 S assembly completely. Thus, p rocessing stem facilitates, but is not essential for assembly. (C) 199 8 Academic Press Limited.