have proposed two possible roles for Bar regulation: 
1) Bar RNA controls the relative levels of specific 
tRNAs to fit the frequency of codon usage in X, 
which assumes that Bar RNA regulates Pth activity 
differentially for distinct peptidyl-tRNAs; and 2) Bar 
RNA acts on termination of polypeptide chains di- 
rected by phage transcripts. 
Among X genes, UGA is the more frequent termina- 
tion codon (and not UAA as in the host cell). In 
addition, it is frequently the case in the X genome 
that the UGA termination codon of a gene partially 
overlaps with the AUG initiation codon of the next 
gene in the tetramer AUGA. Since the Bar core tran- 
script contains AUGA and allegedly interacts with 
ribosomal 1 6S RNA, this suggests a role in polypep- 
tide chain termination/initiation events for the 
overlapping genes. 
Dr. Guarneros is Professor of Genetics and Mo- 
lecular Biology at the Center for Research and Ad- 
vanced Studies, National Polytechnic Institute, 
Mexico City. 
Articles 
Garcia- Villegas, M.R., De La Vega, F.M., Galindo, 
J.M., Segura, M., Buckingham, R.H., and Guar- 
neros, G. 1991. Peptidyl-tRNA hydrolase is in- 
volved in X inhibition of host protein synthesis. 
£MfiO/ 10:3549-3555. 
Murgola, E.J., and Guarneros, G. 1991. Ribosomal 
RNA and peptidyl-tRNA hydrolase: a peptide 
chain termination model for X bar RNA inhibi- 
tion. Biochimie 73:1573-1578. 
MOLECULAR GENETICS OF PHOTOSYNTHESIS AND CARBON ASSIMILATION IN PLANTS 
Luis R. Herrera-Estrella, Ph.D., International Research Scholar 
Photosynthesis and Carbon Assimilation 
Photosynthesis and carbon assimilation are the 
most important biochemical and molecular events 
in the life cycle of higher plants and, indeed, the key 
to the provision of nutrients for the whole food 
chain. Solar energy is first collected in the chloro- 
plasts of photosynthetic tissues, mainly in leaves, by 
light-harvesting antennas composed of chlorophyll 
and protein molecules (chlorophyll fl/fe-binding 
proteins) . The energy obtained is then used to con- 
vert atmospheric CO2 into triose phosphate mole- 
cules. These three-carbon molecules proceed 
through a series of reactions, called the Calvin- 
Benson cycle, that culminates in the production of 
sugars, from which all the organic molecules that 
are required for the life of plants are synthesized. 
Triose phosphate molecules are converted in the 
cytoplasm of photosynthetic or source cells into su- 
crose, which is translocated through the phloem to 
feed all nonphotosynthetic or consumer tissues — 
roots, flowers, seeds, tubers, etc. Assimilated carbon 
is temporally or permanently stored in the form of 
starch in both source and consumer tissues. The 
starch stored in seeds or tubers serves as the major 
source of carbon and energy for the germination and 
development of new plants. 
The light-dependent production of ATP and 
NADPH, the reductive assimilation of CO2, and su- 
crose and starch synthesis are interlinked and inter- 
dependent. In vivo, these processes must be coordi- 
nated at both the biochemical and genetic level 
(i.e., at the level of gene expression). The balance 
between the efficiency of CO2 fixation, sucrose 
translocation and uptake, and assimilation of su- 
crose in consumer tissues plays a fundamental role 
in determining the productivity of any given plant 
species. This balance is affected by both genetic de- 
terminants of the individual and its interaction with 
the environment. 
Characterization of DNA Sequences Involved 
in the Light Regulation and 
Tissue-Specific Expression of cab Genes 
For the past several years. Dr. Herrera-Estrella's 
laboratory, in collaboration with Dr. June Simpson, 
has identified cis-acting sequences involved in the 
regulation of cab and rubisco genes. These se- 
quences have been shown to be responsible for the 
light-inducible and tissue-specific expression of 
both types of photosynthetic genes. Deletion analy- 
sis using transgenic plants has allowed the identifi- 
cation of a 247-bp DNA sequence that can regulate a 
heterologous promoter in a light-inducible and 
tissue-specific fashion and contains positive and neg- 
ative regulatory elements. 
During the past year the effort of this group has 
focused on identifying the trans-acting factors that 
interact with this 247-bp sequence and the DNA 
motif to which these trans-acting factors bind. Two 
protein factors were identified that bind to these 
INTERNATIONAL RESEARCH SCHOLARS 
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