DNA Synthesis and Bacterial Contamination in Palnts
A GC rich satellite DNA is detected in many plant DNA preparations. This satellite is rapidly labeled either with 32-P or 14-C uridine, but, in contrast with bulk DNA, it does not incorporate 14-C thymidine. Its synthesis is triggered very early during seed germination, and is also detected in a variety of circumstances such as hormonal treatments, temperature shifts and wounding. Therefore, it has appeared to be of special significance in the regulation of cell differentiation. Unfortunately, this satellite DNA turns out to be bacterial in origin. Indeed, though seeds were thoroughly surface sterilized before soaking and allowed to germinate on sterile wet filter paper, satellite DNA can be washed off with 2% Na-hypochloride, provided that seed coats were removed. Furthermore, DNA with high specific activity, similar in buoyant density to satellite can be recovered from the radioactive incubation medium. Evaluation of bacterial contamination in hormone treated seedlings reveals that higher is the growth stimulation, higher are the bacterial counts. Some of the contaminant bacteria of radish and broad bean seeds were isolated. Isopycnic centrifugation and preliminary reassociation kinetic data indicate that their DNA and the so-called satellite DNA are very similar, in contrast with true satellite DNA. Most of the isolated bacteria are resistant to currently used antibiotics such as penicillin, chloramphenicol, or colimycin. Streptomycin and tetracyclin efficiently prevent bacteria, but disturb normal germination. Differential labelling is explained by the inability of bacteria to use thymidine, and the size of the phosphate pools in plants. Bacteria were observed between the seed coat and the tissues. Upon germination, they proliferate in the hair root region, where they appeared embedded in mucilageneous compounds, and sticking closely to the cell wall. These observations account for most of the striking features of rapidly labelled plant satellite DNA.