The effects of microparticle size, helium pressure, and target distance on transformation of the uracil auxotroph S. cerevisiae st 948 with the uracil-containing plasmid YEp352 are shown in Table 3. The initial experiment (Table 3, Experiment 1) shows that both M10 and M17 tungsten microparticles are effective in transforming S. cerevisiae, that cells on level 2 are transformed >500-fold more efficiently than cells on level 4, and that changing the helium pressure between 1,100 and 1,550 psi has very little effect on transformation efficiency. A second experiment (Table 3, Experiment 2) confirmed these conclusions, and also suggested that bombardment with M17 tungsten microparticles consistently produced more transformants than did bombardment with M10 tungsten microparticles. Results with 1.0 mm gold and 1.6 mm gold microparticles indicated that the former were approximately equivalent to M10 tungsten microparticles in transformation efficiency, but that larger gold particles had a 100-fold lower transformation rate than the other three types of microparticles. This suggests that the 1.6 mm gold particles, the largest particles used in this experiment with an average diameter of 1.6 mm, probably destroy the cells upon bombardment.
Subsequent experiments with S. cerevisiae used a fractional factorial
design to optimize both gap distance and macrocarrier travel distance
as a function of both helium pressure and target distance. In a fractional
factorial experiment (Kempthorne, 1983) interactions between several factors
are measured at selected points. A statistical model is then used to predict
the optimum condition