The universally conserved core region in the group I intron ribozymes is responsible for its catalytic activity, The structural elements in this region have been known to organize the active site of this class of ribozymes. However, it has been unclear whether all elements are requisite or some elements are dispensable for conducting the catalysis. To investigate the necessity of these elements in the catalysis, we prepared and examined a series of mutants having a nick or deletion in those elements. In this report, we show that two elements, p8 and 5' portion of J8/7, are non-essential for activity. (C) 2000 Academic Press.