Homologous Chromosomal Regions Research Articles

The replicative organization of DNA in polytene chromosomes of Drosophila hydei.

Salivary-gland nuclei ofDrosophila hydei were pulse-labeledin vitro with3H-thymidine and studied autoradiographically in squash preparations. The distribution of radioactive label over the length of the polytene chromosomes was discontinuous in most of the labeled nuclei; in some nuclei the pattern of incorporation was continuous. Comparison of the various labeling patterns of homologous chromosome regions in different nuclei showed that specific replicating units are replicated in a specific order. By combining autoradiography with cytophotometry of Feulgen-stained chromosomes, it was possible to correlate thymidine labeling of specific bands with their DNA content. The resulting data indicate that during the S-period many or perhaps all of the replicating units in a salivary-gland nucleus start DNA synthesis simultaneously but complete it at different times. Furthermore, the data support the hypothesis that the chromomere is a unit of replication or replicon. The DNA content of haploid chromomeres was found to be about 5×10-4 pg for the largest bands inDrosophila hydei. From the results of H3-thymidine autoradiography and Feulgen-cytophotometry on neuroblast and anlage nuclei it was concluded that during growth of the polytenic nucleus heterochromatin is for the most part excluded from duplication. The results of DNA measurements in interbands of polytene chromosomes do not agree with a multistrand structure for the haploid chromatid. A chromosome model is proposed which is in accordance with the reported results and with current views concerning the replicative organization of chromosomes.

GENE TRANSFER BY F' STRAINS OF ESCHERICHIA COLI. IV. EFFECT OF A CHROMOSOMAL DELETION ON CHROMOSOME TRANSFER.

Pittard, James (Yale University, New Haven, Conn.), and T. Ramakrishnan. Gene transfer by F' strains of Escherichia coli. IV. The effect of a chromosomal deletion on chromosome transfer. J. Bacteriol. 88:367-373. 1964.-Evidence is presented that in an F' strain of Escherichia coli. AB1206, the chromosomal region corresponding to the merogenote of F(14) is deleted. AB1206 differs from all other F' strains in its stability and in its relative inability to transfer chromosomal markers under conditions in which transfer of the merogenote occurs at a frequency of 100%. Its stability can be accounted for by the fact that the loss of the F merogenote, which contains approximately 10% of the genetic information of the cell, would be a lethal event. The failure of AB1206 to transfer chromosomal markers, except at very low frequencies, can be explained in terms of the crossover model proposed to explain chromosome transfer by F' strains, since the homologous chromosomal region is missing in this strain. The conclusion that the chromosomal region is deleted is strengthened by the finding that the different enzymes coded for by the genes on F(14) are present in AB1206 at the level found in haploid strains, but have double this activity in homozygous diploids carrying F(14).

SELFER MUTANTS OF SALMONELLA TYPHIMURIUM.

The name selfer has been given to a mutant strain of bacteria in which transduction with homologous phage gives rise to a significantly larger number of wild-type variants than occurs by spontaneous reversion in uninfected control bacteria. Experiments with 201 auxotrophic mutants of Salmonella typhimurium representing 22 gene loci carried in 12 transucing fragments showed that about 40% of these mutants are selfers. In similar experiments with 32 auxotrophs representing four histidine loci, one selfer has been detected. A statistical analysis has revealed that selfer mutations are not distributed at random either within or among the loci where they are known to occur. The frequency of wild- type variants in selfing is evidently connected in some way with the transduction process. The excess variants cannot be transductants, however, since they appear also in crosses between selfer recipients and donors that carry a deletion covering the selfer gene. It is proposed that they are revertants: that the presence of a transducing fragment in the proximity of a selfer gene, when the fragment synapses with the homologous region of the bacterial chromosome, stimulates the gene's mutability. (auth)