VARIABLE REGION V1 OF SACCHAROMYCES-CEREVISIAE 18S RIBOSOMAL-RNA PARTICIPATES IN BIOGENESIS AND FUNCTION OF THE SMALL RIBOSOMAL-SUBUNIT

The role of helix 6, which forms the major portion of the most 5'-loca ted expansion segment of Saccharomyces cerevisiae 18S rRNA, was studie d by in vivo mutational analysis. Mutations that increased the size of the helical part and/or the loop, even to a relatively small extent, abolished 18S rRNA formation almost completely. Concomitantly, 35S pre -rRNA and an abnormal 23S precursor species accumulated. rDNA units co ntaining these mutations did not support cell growth. A deletion remov ing helix 6 almost completely, on the other hand, had a much less seve re effect on the formation of 18S rRNA, and cells expressing only the mutant rRNA remained able to grow, albeit at a much reduced rate. Disr uption of the apical A.U base pair by a single point mutation did not cause a noticeable reduction in the level of 18S rRNA but did result i n a twofold lower growth rate of the cells. This effect could not be r eversed by introduction of a second point mutation that restores base pairing. We conclude that both the primary and the secondary structure of helix 6 play an important role in the formation and the biological function of the 40S subunit.