No abstract

The region of the ultraviolet betxeen about 220 mp and the onset of opacity due to the water absorption band (ca. 185 mp) has recently become accessible to commercially available photoelectric spectrophotometers (see, for example, Kaye') as a result of the development of synthetic quartz optics of high transparency in this range. This region of the spectrum is of especial interest, containing as it does the absorption bands of isolated double bonds, amides, and related compounds, and also the higher energy T-T* transitions of aromatic molecules. A number of results of both analytical and theoretical interest in biochemistry have already been obtained from spectrophotometric studies in the far ultraviolet region, the more obvious examples being with steroids223 and polypeptides and p r~t e i n s .~.~ An examination of the absorption spectra of the nucleic acids and their various components in the far ultraviolet can be expected to be of general interest. RIore specifically, such results should be useful in the theoretical iiwestigations of the origin of the hypochromic effects (see, for example, TinocoG and Rhodes7) and in practical applications such as the estimation of the amount of secondary structure or the degree of denaturation,s the analysis of nucleotide mixturesJg and the identification of the doniinaiit composition of regions of nucleic acid molecules displaying selective denaturationlo which have thus far been limited to the use of measurements in the 260 mp region. This paper consists of a systematic presentation of such spectra and brief comment upon them. EXPERIMENTAL MaterialsPurine and pyrimidine bases, nucleosides, nucleotides, and sugars were obtained from the California Biochemical Corporation; purine, pyrimidine, and ~-ribosed-phosphate from JIann Research Laboratories. According to the manufacturers the samples were chromatographically pure. We are indebted to R. Thach for samples of polynucleotides, to Dr. H. Boedtker for samples of soluble ribonucleic acid (sRSA), and to Drs. J. Marmur and R. Rownd for samples of deoxyribonucleic acid (DKA).

No abstract

Three mutations of the enzyme dihydrofolate reductase were constructed by oligonucleotide-directed mutagenesis of the cloned Escherichia coli gene. The mutations--at residue 27, aspartic acid replaced with asparagine; at residue 39, proline replaced with cysteine; and at residue 95, glycine replaced with alanine--were designed to answer questions about the relations between molecular structure and function that were raised by the x-ray crystal structures. Properties of the mutant proteins show that Asp-27 is important for catalysis and that perturbation of the local structure at a conserved cis peptide bond following Gly-95 abolishes activity. Substitution of cysteine for proline at residue 39 results in the appearance of new forms of the enzyme that correspond to various oxidation states of the cysteine. One of these forms probably represents a species cross-linked by an intrachain disulfide bridge between the cysteine at position 85 and the new cysteine at position 39.

No abstract