118
FRED H. WILT
in older embryos. While the matter is not fully resolved, the lessening
of inhibition of protein synthesis by 8-azaguanine may not simply be
due to permeability changes (cf. Wilt, 1966), and further studies are
required to explain development of refractoriness.
Wilt (1965a) has shown that 8-azaguanine inhibits the incorporation
of uridine-H
3
into all classes of RNA during a 30-min pulse of isotope.
Not only is RNA synthesis inhibited, but several experiments have
shown 8-azaguanine-C
14
is itself incorporated into the RNA of freshly
isolated blastoderms. A good deal of the isotope is associated with the
smaller classes of RNA, presumably the tRNA fraction. This has led to
the proposal that even though 8-azaguanine interferes with nucleic acid
metabolism, it functions in inhibiting protein synthesis in this system
by operating at the translational (cytoplasmic) level. Based on the fact
that incorporation of 8-azaguanine is primarily into a tRNA like fraction, it is proposed that "counterfeit" tRNA which is less effective in
accepting amino acids or in participating in the peptide polymerization
reaction is the basis of 8-azaguanine inhibition. Weinstein and Grünberger (1965) have recently shown that tRNA of B. cereus containing
8-azaguanine functions quite well for polyuridylic acid directed incorporation of amino acids in E. coli. They conclude, however, that their
experiments do not critically rule out the possibility that toxic effects of
the analog may be due to modifications of the tRNA. Levin (1965) has
obtained similar results and also has shown that tRNA containing
8-azaguanine of B. cereus can accept leucine, lysine, alanine, and valine
in unimpeded fashion. The presence of 8-azaguanine does modify the
secondary structure of the tRNA, however. These same tests should be
and are being applied to the early chick embryo system.
Wilt (1962) examined the type of hemoglobin formed in the presence
of 8-azaguanine by immunochemical procedures (agar-gel diffusion
analysis), and although this method was a sensitive tool for detecting
small amounts of hemoglobin, no qualitative differences in the serological
properties of the hemoglobin formed in the presence of the inhibitor
were detected. Hell (1966) has carried out a similar comparison by
using the method of polyacrylamide-gel electrophoresis. Hemoglobins
formed in normal control embryos were compared to hemoglobins formed
in blastoderms treated with 8-azaguanine, BUdR, or X-irradiation ;
although hemoglobin synthesis was depressed by these agents, no obvious
qualitative differences in the type of hemoglobin formed were found.
2. Protein Metabolisjn
Several agents that interfere primarily with protein synthesis have also
been employed, and the results clearly support the notion that the initial
accumulation of hemoglobin depends upon the integrity of protein syn-
FRED H. WILT
in older embryos. While the matter is not fully resolved, the lessening
of inhibition of protein synthesis by 8-azaguanine may not simply be
due to permeability changes (cf. Wilt, 1966), and further studies are
required to explain development of refractoriness.
Wilt (1965a) has shown that 8-azaguanine inhibits the incorporation
of uridine-H
3
into all classes of RNA during a 30-min pulse of isotope.
Not only is RNA synthesis inhibited, but several experiments have
shown 8-azaguanine-C
14
is itself incorporated into the RNA of freshly
isolated blastoderms. A good deal of the isotope is associated with the
smaller classes of RNA, presumably the tRNA fraction. This has led to
the proposal that even though 8-azaguanine interferes with nucleic acid
metabolism, it functions in inhibiting protein synthesis in this system
by operating at the translational (cytoplasmic) level. Based on the fact
that incorporation of 8-azaguanine is primarily into a tRNA like fraction, it is proposed that "counterfeit" tRNA which is less effective in
accepting amino acids or in participating in the peptide polymerization
reaction is the basis of 8-azaguanine inhibition. Weinstein and Grünberger (1965) have recently shown that tRNA of B. cereus containing
8-azaguanine functions quite well for polyuridylic acid directed incorporation of amino acids in E. coli. They conclude, however, that their
experiments do not critically rule out the possibility that toxic effects of
the analog may be due to modifications of the tRNA. Levin (1965) has
obtained similar results and also has shown that tRNA containing
8-azaguanine of B. cereus can accept leucine, lysine, alanine, and valine
in unimpeded fashion. The presence of 8-azaguanine does modify the
secondary structure of the tRNA, however. These same tests should be
and are being applied to the early chick embryo system.
Wilt (1962) examined the type of hemoglobin formed in the presence
of 8-azaguanine by immunochemical procedures (agar-gel diffusion
analysis), and although this method was a sensitive tool for detecting
small amounts of hemoglobin, no qualitative differences in the serological
properties of the hemoglobin formed in the presence of the inhibitor
were detected. Hell (1966) has carried out a similar comparison by
using the method of polyacrylamide-gel electrophoresis. Hemoglobins
formed in normal control embryos were compared to hemoglobins formed
in blastoderms treated with 8-azaguanine, BUdR, or X-irradiation ;
although hemoglobin synthesis was depressed by these agents, no obvious
qualitative differences in the type of hemoglobin formed were found.
2. Protein Metabolisjn
Several agents that interfere primarily with protein synthesis have also
been employed, and the results clearly support the notion that the initial
accumulation of hemoglobin depends upon the integrity of protein syn-