146
HERMAN DENIS
appearance of tissue-specific proteins. This would cause the individuation
of the tissues and organs of the embryo.
It may be useful to compare the possible mechanisms for gene expression in differentiating cells with the model proposed by Jacob and Monod
(1961) for DNA transcription in bacteria. In the bacterial genome the
genes corresponding to the enzymes involved in a given metabolic pathway would be placed side by side and their expression would be under
the control of a special gene, called "operator." The genes controlled by
the same operator would be transcribed in the form of a large messenger
molecule, which would carry all the information necessary for the synthesis of the enzymes belonging to the same metabolic chain (Kiho and
Rich, 1965). The coordinate appearance of these enzymes would thus be
determined in a relatively simple way. The transcription of the mRNA
and, hence, the synthesis of the enzymes is apparently switched on and
off by low-molecular-weight substances (inductors) which act on the
operator through an intermediate agent, called "repressor."
A mechanism comparable to that proposed by Jacob and Monod for
gene expression in bacteria does not seemed to be functional in embryonic
cells, since no clear-cut case of enzyme induction has ever been observed
in growing embryos. There might, however, be some analogy between the
mechanism that controls the transcription of the genome in bacteria and
the mechanism that is responsible for cell differentiation in multicellular
organisms. Messenger RNA might be synthesized in embryonic development in a polycistronic form, i.e., in a form that carries the information
necessary for the synthesis of several proteins (Brown and Littna,
1966a). Thus the information corresponding to the many proteins and
enzymes that concomitantly appear with the process of differentiation
might be transcribed in one step and transferred as a whole to the
cytoplasm. It is conceivable that this transcription is, in turn, controlled
by one or several operators. How these operators could further be
activated or repressed by cytoplasmic factors remains so far a matter
of speculation.
It is tempting to imagine that some regulatory function in DNA
transcription might be fulfilled by the short-lived mRNA that is continuously synthesized by differentiated embryos (Fig. 19). It is difficult to understand why the differentiated cells need to resort continuously to the genetic information enclosed in their nucleus, since the
mRNA responsible for the synthesis and for the renewal of their proteins
seems to have been transcribed once for all and conserved in a stabilized
form. The unstable mRNA present in differentiated cells might itself be
HERMAN DENIS
appearance of tissue-specific proteins. This would cause the individuation
of the tissues and organs of the embryo.
It may be useful to compare the possible mechanisms for gene expression in differentiating cells with the model proposed by Jacob and Monod
(1961) for DNA transcription in bacteria. In the bacterial genome the
genes corresponding to the enzymes involved in a given metabolic pathway would be placed side by side and their expression would be under
the control of a special gene, called "operator." The genes controlled by
the same operator would be transcribed in the form of a large messenger
molecule, which would carry all the information necessary for the synthesis of the enzymes belonging to the same metabolic chain (Kiho and
Rich, 1965). The coordinate appearance of these enzymes would thus be
determined in a relatively simple way. The transcription of the mRNA
and, hence, the synthesis of the enzymes is apparently switched on and
off by low-molecular-weight substances (inductors) which act on the
operator through an intermediate agent, called "repressor."
A mechanism comparable to that proposed by Jacob and Monod for
gene expression in bacteria does not seemed to be functional in embryonic
cells, since no clear-cut case of enzyme induction has ever been observed
in growing embryos. There might, however, be some analogy between the
mechanism that controls the transcription of the genome in bacteria and
the mechanism that is responsible for cell differentiation in multicellular
organisms. Messenger RNA might be synthesized in embryonic development in a polycistronic form, i.e., in a form that carries the information
necessary for the synthesis of several proteins (Brown and Littna,
1966a). Thus the information corresponding to the many proteins and
enzymes that concomitantly appear with the process of differentiation
might be transcribed in one step and transferred as a whole to the
cytoplasm. It is conceivable that this transcription is, in turn, controlled
by one or several operators. How these operators could further be
activated or repressed by cytoplasmic factors remains so far a matter
of speculation.
It is tempting to imagine that some regulatory function in DNA
transcription might be fulfilled by the short-lived mRNA that is continuously synthesized by differentiated embryos (Fig. 19). It is difficult to understand why the differentiated cells need to resort continuously to the genetic information enclosed in their nucleus, since the
mRNA responsible for the synthesis and for the renewal of their proteins
seems to have been transcribed once for all and conserved in a stabilized
form. The unstable mRNA present in differentiated cells might itself be