ribozyme RNA with enzymatic or catalytic ability
specifically to cleave (break down) or bind RNA
molecules. Also known as autocatalytic or catalytic
RNA.
ribulose-1,5-bisphosphate carboxylase/oxygenase
(rubisco) A magnesium-dependent ENZYME. The pri-
mary enzyme of carbon dioxide fixation in plants and
AUTOTROPHICbacteria. It catalyzes the synthesis of 3-
phospho-D-glycerate from ribulose bisphosphate and
also the oxidation of ribulose bisphosphate by O 2 to 3-
phospho-D-glycerate and 2-phosphoglycolate.Rieske iron-sulfur protein An IRON-SULFUR PRO-
TEIN of the MITOCHONDRIAL respiratory chain, in
which the [2FE-2S] CLUSTERis coordinated to two sul-
fur LIGANDs from cysteine and two imidazole ligands
from histidine. The term is also applied to similar pro-
teins isolated from photosynthetic organisms and
microorganisms and other proteins containing [2Fe-2S]
clusters with similar COORDINATION.
See alsoPHOTOSYNTHESIS.Ritchie equation The LINEAR FREE-ENERGY RELATION
log kN= log k 0 + N+
applied to the reactions between nucleophiles and cer-
tain large and relatively stable organic cations, e.g.,
arenediazonium, triarylmethyl, and aryltropylium
cations in various solvents. kNis the rate constant for
reaction of a given cation with a given nucleophilic sys-
tem (i.e., given nucleophile in a given solvent); k 0 is the
rate constant for the same cation with water in water;
and N+is a parameter that is characteristic of the nucle-
ophilic system and independent of the cation. A surpris-
ing feature of the equation is the absence of a coefficient
of N+, characteristic of the substrate (this is the s in the
SWAIN-SCOTT EQUATION), even though values of N+vary
over 13 log units. The equation thus involves a gigantic
breakdown of the REACTIVITY-SELECTIVITY PRINCIPLE.
The equation has been extended both in form and in
range of application.RNA SeeRIBONUCLEIC ACID.roentgen A measure of ionizing radiation named
after Wilhelm Roentgen, a German scientist who dis-
covered X rays in 1895. One roentgen is the amount of
gamma or X rays needed to produce ions resulting in a
charge of 0.000258 coulombs/kilogram of air under
standard conditions.rotamer One of a set of conformers (one of a set of
stereoisomers, each of which is characterized by a CON-rotamer 239Transmission electron micrograph of ribosomes and polysomes
from chick embryo fibroblast cells. Ribosomes are the site of pro-
tein synthesis in the cell. Each consists of a large and small sub-
unit made of roughly equal ratios of ribosomal RNA (ribonucleic
acid) and 40 or more different types of protein. During protein syn-
thesis, a molecule of messenger RNA attaches to a groove in the
ribosome and slides along. As it passes, amino acids are bound
together in an order spelt out by the base code on the messenger
RNA. Groups of ribosomes attached to the same messenger RNA
molecule are called polysomes. Magnification ×135,000 at 10 ×8-
inch size.(Courtesy of Dr. Gopal Murti/Science Photo Library)