Larval age and the pattern of DNA synthesis in polytene chromosomes

It has been shown, by autoradiography using H3-thymidine, that the frequency of salivary gland cells where DNA synthesis covers the entire length of a specific polytene chromosomal segment (Chromosome II, 56F-60A, Drosophila melanogaster) drops off some time during the last day of larval life. The frequency of highly discontinuous DNA synthesis over the same region remains at about the same level until a stage closer to puparium formation, when all DNA synthesis stops.If a cycle of DNA synthesis, once initiated, goes to completion, then this finding indicates that the patterns of spatially continuous synthesis tend to occur early in the replicative cycle and that the terminal phases of the cycle involve highly discontinuous patterns of synthesis.

The effect of feeding with a tryptophan-free amino acid mixture on rat-liver polysomes and ribosomal ribonucleic acid

1. Starving rats were given complete and tryptophan-deficient amino acid mixtures by stomach tube and were killed from 1 to 7hr. later. The polysome profile in the livers of rats fed with the tryptophan-deficient mixture showed a shift in distribution such that the large aggregates were decreased and the small aggregates were increased, particularly dimers. This polysome shift was reversed when the complete amino acid mixture was given by stomach tube 2hr. after administering the tryptophan-free amino acid mixture. 2. After removal of liver polysomes by centrifugation, some smaller ribosomal aggregates (oligosomes) remaining in suspension were harvested by prolonged centrifugation of the supernatant fluid. A large increase in the dimer population of this fraction was observed in the rats receiving the incomplete mixture. 3. When the polysome and oligosome fractions were incubated with cell sap, an energy-generating system and labelled amino acids dl-[1-(14)C]leucine and l-[Me-(14)C]tryptophan were incorporated into the cell fractions in the ratio 4.5:1. Preparations of polysomes and oligosomes from rats fed with the tryptophan-free amino acid mixture showed a decreased amino acid-incorporating activity compared with particulate preparations made from rats fed with the complete mixture. 4. The yield of free ribosomes prepared from the unfractionated liver microsomes by treatment with iso-octane was 40-50% greater in rats fed with the amino acid mixture deficient in tryptophan. 5. A post-microsomal fraction was prepared from cell sap and was shown to consist of ribosomal sub-units. When the animals were fed with the tryptophan-deficient mixture, there was an increase in content of this post-microsomal fraction and in the ratio 30s RNA/19s RNA. Rats were also given [5-(3)H]orotic acid at the time of feeding with the amino acids. Lack of tryptophan in the mixture caused a decrease in the specific activity of both RNA fractions which affected the 30s RNA more extensively than the 19s RNA. 6. These changes in the distribution and quantity of the cellular components engaged in protein synthesis are discussed in relation to RNA metabolism and amino acid-incorporating activity of the liver cell and their response to feeding with the tryptophan-free amino acid mixture.