Interaction of nucleic acids with lipid membranes

The thermodynamics of nucleic acids which were enclosed in reverse-phase evaporation vesicles was studied by thermal denaturation with optical recording. The denaturation curves were recorded with a dual wavelength spectrophotometer. The sum of the hypochromicity of the nucleic acid and of the change in turbidity of the vesicles was measured at 260 nm and was corrected for the change in turbidity at 320 nm. Cloned fragments of double-stranded DNA containing 180 base pairs and poly A:poly U were enclosed in REV with a yield up to every vesicle containing five nucleic acid molecules. Vesicles were prepared from egg-lecithin, and the surface charge of the vesicles was varied by addition of stearic acid, phosphatidyl-glycerol and phosphatidyl-serine. The helix-coil transition of the nucleic acid enclosed in the vesicle could be resolved from that of the free nucleic acid. Due to the enclosure into the egg-lecithin REV the transition is stabilized from 70.5 degrees to 74 degrees C, the transition is broadened from 0.7 degrees C to 2.7 degrees C. Varying the phosphatidyl-serine-lecithin-ratio from 0-100%, an optimum in the yield of enclosure at 20% was obtained, a further broadening of the transition to 5.5 degrees C and a decrease of the stabilization down to a small destabilization at 100% phosphatidyl serine was observed. Qualitatively, similar effects were observed with poly A:poly U. Variation of the ionic strength led to the conclusion that the replacement of the counterions of the phosphate backbone by the surface charge of the membrane, as well as a direct contact between the nucleic acid and the membrane have to be assumed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Primary structure of Escherichia coli ribosomal protein L31.

Protein L31 from the 50S ribosomal subunit of Escherichia coli was manually sequenced by the dansyl-Edman method. Owing to the availability of only small quantities of purified L31, sequencing methods were scaled down such that the entire primary structure could be determined with 700 microgram of protein. The techniques employed are described in detail. The protein consists of a single chain of 62 amino acids, with a calculated molecular weight of 6967. Four half-cystine residues were identified at positions 16, 18, 37, and 40. Evidence is presented that suggests that these residues form two disulfide bridges in the protein, as isolated.