NOVEL REQUIREMENTS IN PERIPHERAL STRUCTURES OF THE EXTENDED SATELLITE-2 HAMMERHEAD

A number of RNAs that infect plants self-cleave using a domain called the hammerhead. The consensus plant hammerhead has three base paired s tem structures, Stems I and III flank the cleaved phosphodiester and a re connected to stem II by two unpaired and highly conserved sequences . We demonstrated previously that satellite 2 transcripts from the new t, Notophthalmus viridescens, use a modified hammerhead structure for self-cleavage. Here we show that hammerheads with similar modification s occur in satellite 2 from species representing two additional famili es of salamanders (Ambystoma talpoideum and Amphiuma tridactylum). The distinctive features of the modified satellite 2 hammerheads include unusually short stem III regions and internally looped extensions to s tem I that are required for cleavage, However, despite the strict nucl eotide requirement in the internal stem I loop of the newt hammerhead, the details of the stem I extensions differ in the three salamanders. An analysis of chimeric transcripts indicated that, in order for a sp ecific stem I extension to be functional, it must be compatible with o ther regions of the modified satellite 2 hammerhead, One such region i s stem II, and a mutational analysis has confirmed that there are spec ific sequence and/or structural requirements in stem II.