Isolation and detailed characterization of human cell lines resistant to -threo-chloramphenicol

The isolation and characterization of chloramphenicol resistant derivatives of the human cell line HeLa B is described. Growth of resistant lines was unaffected in the presence of 100 μg/ml -threo-chloramphenicol, whereas growth of the parental cells was inhibited at 12.5 μg/ml. The incorporation of [³⁵S]methionine into mitochondrial protein of intact resistant cells continued normally in the presence of 100 μg/ml chloramphenicol (cytoplasmic protein synthesis was blocked by addition of 50 μg/ml emetine). Under these conditions the electrophoretic profile of labelled, presumptive mitochondrially-made proteins was similar to that of the parental cell line labelled in the absence of chloramphenicol. The cell lines selected in the presence of chloramphenicol also showed increased resistance to some other inhibitors of mitochondrial protein synthesis, e.g. carbomycin and mikamycin. [¹⁴C]Chloramphenicol was found to have normal access to the interior of resistant cells and it is therefore unlikely that resistance results from altered cell permeability. No modification of the drug by acetylation or glucuronide conjugation mechanisms was observed. The possibilities remain that resistance is mediated by altered permeability of the mitochondrial membrane, or from modification to a component of the mitochondrial protein synthetic system.     

Lithium transport across isolated frog skin epithelium

Summary Transepithelial Li⁺ influx was studied in the isolated epithelium from abdominal skin ofRana catesbeiana. With Na⁺-Ringer's as inside medium and Li⁺-Ringer's as outside medium, the Li⁺ influx across the epithelium was 15.6 A/cm². This influx was considerably reduced by removal of either Na⁺ or K⁺ from the inside bath or by the addition of ouabain or amiloride. Epithelial K⁺ or Na⁺ concentration was respectively lower in epithelia bathed in K⁺-free Ringer's or Na⁺-free Ringer's. In conditions of negligible Na⁺ transport, a 20mm Li⁺ gradient (outin) produced across the short-circuited epithelium a Li⁺ influx of 11.8 A/cm² and a mean short-circuit current of 10.2 A/cm². The same Li⁺ gradient in the opposite direction produced a Li⁺ outflux of only 1.9 A/cm². With equal Li⁺ concentration (10.3 and 20.6mm) on both sides of the epithelium, plus Na⁺ in the inside solution only, a stable Li⁺-dependent short-circuit current was observed. Net Li⁺ movement (outin) was also indirectly determined in the presence of an opposing Li⁺ gradient. Although Li⁺ does not substitute for Na⁺ as an activator of the (Na⁺+K⁺)-ATPase from frog skin epithelium, Li⁺ influx appears to be related to Na⁺–K⁺ pump activity. It is proposed that the permeability of the outer barrier to Na⁺ and Li⁺ is regulated by the electrical gradient produced by electrogenic Na⁺–K⁺ pumps located in the membrane of the deeper epithelial cells.     

The Isolation and Characterization of Adenosine Monophosphate-rich Polynucleotides Synthesized by Soybean Hypocotyl Cells: Their Relation to Messenger Ribonucleic Acid

Plant ribonucleic acids which have high adenosine monophosphate concentrations were studied. Purified deoxyribonucleic acid-like ribonucleic acid and tenaciously bound ribonucleic acid fractions both contained poly-adenosine monophosphate sequences (those from the latter being longer than those from the former); without these poly-adenosine monophosphate sequences their base compositions were the same. The average poly-adenosine monophosphate sequence from purified tenaciously bound ribonucleic acid was 160 residues long, as measured by gel electrophoresis. However, base hydrolysis and chromatography indicated one 3′-nucleoside (adenosine) per 71 nucleotides, giving a chain length of 72 residues. The dominant species in the cytoplasm, as measured by radioactive precursor incorporation, was tenaciously bound ribonucleic acid, whereas deoxyribonucleic acid-like ribonucleic acid was present in greater amounts in the nucleus. This work provides evidence that deoxyribonucleic acid-like ribonucleic acid and tenaciously bound ribonucleic acid represent forms of messenger ribonucleic acid in soybean, with deoxyribonucleic acid-like ribonucleic acid residing in the nucleus, perhaps as the messenger ribonucleic acid precursor, and tenaciously bound ribonucleic acid residing, as the active messenger ribonucleic acid, in the cytoplasm.