Glycogen metabloism in the developing chick glycogen body: functional significance of the direct oxidative pathway.

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were quantitatively determined for the first time in glycogen body tissue from late embryonic and neonatal chicks. For comparative purposes, the activities of these enzymes were examined also in liver and skeletal muscle from pre- and post-hatched chicks. The present data show that both the embryonic and neonatal glycogen body lack glucose-6-phosphatase, but contain relatively high levels of glucose-6-phosphate dehydrogenase. The activity of each dehydrogenase in either embryonic or neonatal glycogen body tissue is two- to five-fold greater than that found in muscle or liver from pre- or post-hatched chicks. The relatively high activities observed for both dehydrogenases in the glycogen body, together with the absence of glucose-6-phosphatase activity in that tissue, suggest that the direct oxidative pathway (pentose phosphate cycle) of glucose metabolism is a functionally significant route for glycogen utilization in the glycogen body. It is hypothesized that the glycogen body is metabolically linked to lipid synthesis and myelin formation in the central nervous system of the avian embryo.     

The postembryonic transformation of the shell in emydine box turtles

A key trend in the 210‐million‐year‐old history of modern turtles was the evolution of shell kinesis, that is, shell movement during neck and limb retraction. Kinesis is hypothesized to enhance predator defense in small terrestrial and semiaquatic turtles and has evolved multiple times since the early Cretaceous. This complex phenotype is nonfunctional and far from fully differentiated following embryogenesis. Instead, kinesis develops slowly in juveniles, providing a unique opportunity to illustrate the postembryonic origins of an adaptive trait. To this end, we examined ventral shell (plastral) kinesis in emydine box turtles and found that hatchling plastron shape differs from that of akinetic‐shelled relatives, particularly where the hinge that enables kinesis differentiates. We also demonstrated shape changes relative to plastron size in juveniles, coinciding with a shift in the carapace‐plastron structural connection, rearrangement of ectodermal plates, and bone repatterning. Furthermore, because the shell grows larger relative to the head, complete concealment of the head and extremities is only achieved after relative shell proportions increase. Structural alterations that facilitate the box turtle's transformation are probably prepatterned in embryos but require function‐induced changes to differentiate in juveniles. This mode of delayed trait differentiation is essential to phenotypic diversification in turtles and perhaps other tetrapods.     

Inhibition of peptide initiation by a low molecular weight RNA from rabbit reticulocytes

A heat-stable inhibitor of protein synthesis has been isolated from the postribosomal supernatant of rabbit reticulocytes. Its activity is not susceptible to protease treatment but is destroyed by incubation with alkali. Inhibitory activity can be quantitatively recovered in the aqueous phase after phenol extraction and has the ultraviolet absorption spectrum of a nucleic acid. It is concluded that the inhibitor is RNA. The inhibitory activity sediments in the range of 3 S, but it has not been demonstrated whether the inhibitor RNA is a single molecular species. The inhibitory RNA does not affect peptide elongation but rather blocks a step of peptide initiation. It does not interfere with the formation of the ternary complex between initiation factor 2, GTP, and methionyl-tRNAMetf and does not activate a protein kinase phosphorylating initiation factor 2. The inhibitory RNA appears to be a novel type of RNA that inhibits polypeptide initiation at a step involving ribosomal subunits.     

Cytoskeleton: Anatomy of an organizing center

One component of the yeast spindle pole body, Spc42p, has been found to form a crystalline array within one of the central layers of the structure; the Spc42p crystal might provide a scaffold around which the spindle pole body is assembled, and could be involved in regulating the size of the spindle pole body.