Deletion of internal sequence on the HDV-ribozyme: elucidation of functionally important single-stranded loop regions.

In elucidating functionally important single-stranded loop regions derived mainly from three models in genomic hepatitis delta virus (HDV) ribozyme possessing self-cleavage activity, we have constructed several internal deletion variants of the HDV133 molecule (654-786 nt on genomic RNA) by oligonucleotide-directed mutagenesis. When self-cleavage activities were compared among variants, the HDV133DI-1 (deletion of 701-718 nt) and HDV133DI-3 (deletion of 740-752 nt) ribozyme could maintain their self-cleavage activity, despite at reduced level. However, the activity could be regained in both mutants by some extent under partially denaturing conditions. These results suggest that the above two single-stranded RNA loop regions in HDV ribozyme are not part of the catalytic core but might be involved in the stability of the molecule. In contrast, deletion mutants such as HDV133DI-2 (deletion of 696-722 nt), HDV88DI-1 (deletion of 701-718 nt), HDV88DI-2 (deletion of 696-722 nt), and HDV88DI-4 (deletion of 733-760 nt) abolished catalytic activity. These results suggest that the remaining single-stranded regions of bases between 726-731 and 762-766 in the HDV88 ribozyme may be the potential regions to interact with Mg2+ ions.