Oc mutations in the operators of bacteriophage lambda have been used to analyze the functional organization of the operators. In each operator, repressor binding sites 1 and 2, as identified biochemically, were found to be primarily responsible for the repressor affinity of the operators in vitro and for the repression of lytic functions in vivo. In addition, both sites were shown to be involved in the action of cro product at the operators. The data obtained have been used to estimate the repressor affinities of the individual binding sites. These affinities suggest that repressor bound at OR1 and OR2 interacts cooperatively. The results obtained support a model for repression of the early lambda operons where repressor bound at binding sites 1 and 2 interferes with RNA polymerase binding to the promotor sites.
Manipulation of exogenous auxin and cytokinin levels during sequential subculture in vitro has been used to induce caulogenesis in several long-term tobacco cell lines. Concurrently, tissue samples at various stages of caulogenesis have been examined for nuclear DNA content. While a variety of hormone regimes permitted caulogenesis, extremely high (122.95 μM) cytokinin levels and extremely low (0.285 μM) auxin levels generally gave optimal response. For three lines, caulogenesis was accompanied by a progressive decrease in nuclear DNA content beyond that due to the loss of polyploid cells. In one line, however, DNA content remained stable during regeneration, perhaps reflecting the acquisition of a stably adapted aneuploid karyotype. Both caulogenic response and amount of nuclear DNA were affected by changes in the culture medium. The progressive nature of the observed changes in DNA content is inconsistent with a single-step selection for euploid competent cells. Alternative models postulating either progressive selection for euploidy, or the regulation of karyotype are proposed to explain the results.
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