Nature of a Rapidly Labeled RNA Fraction Described in Higher Plant Systems

Abstract

When seedling tissues are exposed to 82PO4 for short periods a rapidly labeled RNA fraction is frequently observed which is eluted from MAK columns as peak I between the 2 components of the so-called soluble RNA, e.g. in peanut cotyledons (5), in soybean hypocotyls (11), and in different parts of pea and bean seedlings (9). This 32P-labeled fraction is characterized by a high GMP-CMP content. The radioactivity peak of the DNA-RNA fraction which preceeds the optical density profile is also rich in GMP-CMP (4,9,17) whereas the overall base composition of the DNA-RNA fraction has a low GMP-CMP content. Several authors have been aware of the possibility that bacterial contamination could influence the results. By actual bacterial counts (19) and by the use of inhibitors (9, 11) it has been deduced that the microorganisms do not play a major role in producing the observed labeling pattern. It is, however, striking that in experiments with ase-ptic higher plant systems (13,15,) there is a perfect matching of the radioactivity and the OD of the DNA-RNA peak. In watermelon seedlings ( Citrullus vulgaris Schrad.) we find only 4 species of Pseudomonas (kindly identified by Dr. Neidhardt's group) as bacterial contaminants. To check the effect of bacterial contamination upon the 32PO4 incorporation into nucleic acids by the cotyledons, a bacteria-free system of watermelon seedlings was first obtained. During the exposure to 32po4, a culture of the bacteria mentioned above was added back to the incubation medium. The labeling of the nucleic acids in the washed cotyledons was then compared to that in the bacteria-free control. It is shown that both the labeled fractions mentioned above appeared only in the contaminated samples.