...And in the form of catalytic RNA.

In another anti-gene approach, scientists make their own ribozymes--bits of catalytic RNA that chew up other strings of this nucleic acid (SN: 12/22&29/90, p.390). They design each ribozyme to target a specific messenger RNA, usually an RNA generated by mutated genes or oncogenes.

At Mount Sinai School of Medicine in New York City, Hiroyuki Kobayashi has made a particular "hammerhead" ribozyme, so named because of its molecular shape. It seeks out the RNA for a protein that pumps anticancer drugs out of tumor cells, making them resistant to medication. Tests in cells grown in the laboratory show that the ribozyme does chop up this RNA.

To improve on nature's ribozymes, one research team has substituted DNA for some of the RNA in these molecules. The DNA portion recognizes the base-pair sequence of the target RNA, and the RNA part breaks the sequence apart, explains John J. Rossi of the Beckman Research Institute of the City of Hope in Duarte, Calif. He and his colleagues expect these chimeric ribozymes to last longer and destroy RNA more efficiently than pure ribozymes. So far, one has proved six times more efficient than its all-RNA counterpart, says Rossi. He and his co-workers are testing the ability of these molecules to get rid of cancer-promoting gene products found in the bone marrow of people with chronic myelogenous leukemia.