RNA Viral Genetics 
and absence of bound wild-type and mutant po- 
liovirions. In this way we are exploring the spe- 
cific protein-protein contacts made betu^een the 
viruses and the receptor during binding and the 
subsequent conformational changes during entr}' 
of the virus into the cell. 
We have extended the study of RNA genetics to 
she interactions between yeast cells and the small 
double-stranded RNA genome of L-A, a cytoplas- 
mic virus-like particle. Yeast cells, unlike the pri- 
mate cells in which poliovirus and other RNA vi- 
ruses of medical interest are propagated, are 
amenable to elegant genetic analysis, making it 
possible to identif\' quite quickly the cellular 
molecules that are involved in any given process. 
The virus-like particle L-A has been shown by the 
laboratory^ of Reed Wickner (National Institutes 
of Health) to replicate in the cytoplasm of yeast 
cells in large numbers. Genetic analysis of L-A, 
however, will depend on our ability to make de- 
fined mutations in its RNA genome. 
In the past year, we have developed the techni- 
cal capabilities to pursue this goal. First, we had 
to develop the technology to introduce RNA di- 
rectly into yeast cells, which we accomplished by 
using RNA molecules encoding the luciferase 
protein of fireflies. Second, we needed an assay 
for the successful introduction of the L-A genome 
into yeast. Since the particle is so ubiquitous, no 
phenotype for its presence or absence in a yeast 
cell had been reported. We found that, under 
somewhat unusual laboratory conditions, certain 
yeast strains require the L-A genome for growth! 
This suggests the possibility of mutualistic rela- 
tionships between host cells and virus-like RNA 
genomes, a possibility we are also pursuing in 
other systems. 
Most importantly for this work, we now have a 
phenotype for the presence of the L-A RNA ge- 
nome in yeast cells. We have recently been able 
to initiate L-A "infections" in yeast by introduc- 
ing RNA made in vitro. We are now in a position 
to mutate this RNA genome, to study any defect in 
the L-A replication cycle caused by the mutations, 
and to observe the effects of yeast mutants on the 
replication cycle of mutant and wild-type RNA. 
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