Investigation of the mechanism underlying the inhibitory effect of heterologous ras genes in plant cells

The location of untranscribed DNA sequences within ras genes essential for eliciting plant growth suppression

Three heterologous ras DNA-coding sequences and their deletion derivatives were introduced into plant cells to investigate the role of the ras-coding sequences, especially conserved regions, in eliciting growth inhibition. All three ras-coding sequences caused a similar inhibition of plant cell growth, and it was the conserved coding regions which were responsible for this inhibitory effect. The 493 bp conserved region within the v-Ha-ras-coding sequence was studied further, and was shown to be responsible for the inhibitory effect. This region is conserved (over 44%) among the three ras genes studied and encodes a catalytic region of the Ras protein. Small deletions at either the 5' or 3' end of this 493 bp sequence could abolish or dramatically reduce the inhibitory effect. A 36 bp region at the 5' end of the 493 bp region was found to be highly conserved between v-Ha-ras and eight different plant ras or ras-related genes based upon analysis of published sequences. Small deletions affecting this highly conserved 36 bp region completely abolished the inhibitory effect, while deletion of a similar number of base pairs in adjacent regions did not. These results indicate that plant growth inhibition by ras DNA requires small regions at both ends of the 493 bp conserved region.