cules are then sequenced and compared with the 
original parental ribozyme. 
Experiments in progress are directed toward the 
generation of improved ribozyme ligases specifi- 
cally suited for binary probe assays. A threefold im- 
provement in ligation efficiency seems an attainable 
goal and may permit the implementation of RNA 
probe amplification assays capable of detecting as 
few as 200 molecules of target. 
Dr. Lizardi is Professor of Biochemistry at the 
Biotechnology Institute, National Autonomous 
University of Mexico, Cuernavaca. 
EARLY DEVELOPMENT IN NEMATODES 
James D. McGhee, Ph.D., International Research Scholar 
Dr. McGhee's laboratory focuses on the question 
of how specific genes become expressed only in 
certain lineages of the developing embryo. The 
model organism under study is the nematode Caeno- 
rhabditis elegans, a small free-living worm that has 
numerous experimental advantages, such as a de- 
fined cell lineage, small number of cells, and acces- 
sibility to both micromanipulation and manipula- 
tion by classical genetics. The laboratory has 
recently begun to study other nematodes as well — 
for example, the reasonably closely related nema- 
tode Caenorhabditis briggsae and Ascaris suum, 
the evolutionarily distant intestinal parasite of pigs. 
Nematodes are a fascinating group of organisms in 
which to study patterns of gene expression, since 
their basic body plan and developmental strategies 
have remained largely unchanged over hundreds of 
millions of years of evolution, even in the face of 
large changes in DNA sequence. 
Most of the work in the laboratory involves gut 
development, since the nematode intestine is a par- 
ticularly simple lineage. It is established when the 
embryo has only eight cells, and it appears to de- 
velop in the absence of major interactions with 
other cells. As an experimentally convenient molec- 
ular marker of gut differentiation, the animal uses an 
intestine-specific digestive enzyme, the product of a 
carboxylesterase gene called ges-1, which is homol- 
ogous to esterases present in insects and mammals. 
The experimental approach most used at present is 
to modify the cloned ges-1 DNA in vitro, inject it 
into mutant worms that do not express esterase activ- 
ity, and then stain these transformed worms to see in 
which cells the ges- 1 activity appears. Past work has 
shown that ges- 1 control is unexpectedly complex. 
In particular, the gene appears capable of being ex- 
pressed in certain nongut lineages. This nongut ex- 
pression is revealed when specific regions of ges- 1 
have been deleted. The working model in the labora- 
tory is that lineage-specific gene expression in- 
volves not only activators in the expressing lineage 
(in this case, the gut) but also repressors in nonex- 
pressing tissues (in this case, primarily muscle cells 
of the pharynx) . 
Efforts are now focused on identifying the sites of 
action of these regulatory molecules, both putative 
repressors and putative activators. One approach 
that has been used is to investigate the gene se- 
quences and gene control mechanisms in other nem- 
atodes. To this end, the ges-1 homologue has been 
cloned from the related nematode C. briggsae. Cod- 
ing sequences have been highly conserved; but 
apart from the coding regions, except for one short 
sequence element, little obvious similarity can be 
detected. Yet, despite these sequence diff^erences, 
and although the two worms diverged an estimated 
40 million years ago (and possibly even earlier), the 
C. elegans gene appears to be expressed correctly 
when transformed into C. briggsae and, to a good 
approximation, the C. briggsae gene appears to be 
expressed correctly when transformed into C. ele- 
gans. Detailed sequence examinations, deletions, 
and substitutions are now being used to define func- 
tional sites unambiguously. 
The laboratory has also established an experimen- 
tal system in which to investigate the biochemistry 
of early C. elegans development. Parent worms are 
grown in the presence of the deoxynucleotide ana- 
logue fluorodeoxyuridine, and development of the 
next generation of embryos is completely blocked at 
a point in development just after ges- 1 expression 
has been initiated. Protein binding factors have been 
detected in nuclear extracts prepared from this ho- 
mogeneous population of embryos. Two factors 
have been identified that interact with a region of 
the ges-1 gene believed to be the "gut activator," 
and a different factor has been identified that binds 
to the one sequence highly conserved between C. 
elegans and C. briggsae. 
Reasonably large quantities of unfertilized oo- 
520 
