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S R Casjens

Publications and source records attributed to S R Casjens.

12 recordsLinked to original sources
PubMed1993

A programmed translational frameshift is required for the synthesis of a bacteriophage lambda tail assembly protein.

Two proteins, one of 31 kDa and one of 16 kDa, are encoded by a segment of the phage lambda tail gene region that contains two overlapping reading frames, neither of which is long enough to encode the larger protein. We show that the abundant 16-kDa protein (gpG) is encoded by the upstream open reading frame, gene G. The 31-kDa protein, gpG-T, is encoded jointly by gene G and the overlapping downstream T open reading frame. gpG-T is synthesized as the result of a translational frameshift that occurs when a ribosome translating the G gene slips back by one nucleotide at a position six codons from the C terminus of the gene and thereby bypasses the G termination codon to continue on in the T open reading frame. The resulting protein shares 135 residues of N-terminal amino acid sequence with gpG, followed by 144 amino acid residues of unique sequence. The frameshift event occurs with a frequency of approximately 4% at the sequence G GGA AAG, which encodes the dipeptide -Gly-Lys- in both the zero and -1 reading frames. The frameshift frequencies of point mutants in this "slippery sequence" argue that codon-anticodon interactions with both the glycyl and the lysyl-tRNA are important for frameshifting to occur. We find no clear evidence for a pausing mechanism to enhance frameshifting, as is seen in other well-characterized frameshifts. No simple secondary structure has been predicted for the region downstream from the slippery sequence, but this downstream sequence does contribute to the frameshifting rate. Our results together with those of Katsura and Kühl show that the frameshift product, gpG-T, has an essential role in lambda tail assembly, acting prior to tail shaft assembly. The role of gpG in tail assembly is not known. We find that both gpG and the gpG-T are absent from mature virions.

Amino Acid Sequence↗
PubMed1992

Secondary structure and interactions of the packaged dsDNA genome of bacteriophage P22 investigated by Raman difference spectroscopy.

Vibrational spectra of the double-stranded DNA genome of bacteriophage P22 in packaged and unpackaged states are compared by digital difference Raman spectroscopy. The difference Raman spectrum, which is sensitive to structural changes at the level of < 2% of a given nucleotide type, reveals the effects of packaging upon sugar pucker, glycosyl orientation, phosphodiester geometry, base pairing, base stacking, and the electrostatic environment of DNA phosphate groups. For both packaged and unpackaged states, the experiments were performed on aqueous solutions at 25 degrees C containing effective P22 DNA concentrations of 30-50 mg/mL in 200 mM NaCl + 10 mM MgCl2 + 10 mM Tris at pH 7.5. At the experimental conditions employed, the B-form secondary structure of unpackaged P22 DNA is minimally perturbed by packaging the viral genome in the virion capsid. However, the electrostatic environment of DNA phosphates is dramatically altered with packaging. Specifically, we find the following: (1) C2'-endo sugar pucker and anti glycosyl orientations are conserved for all nucleosides. (2) Watson-Crick base pairing is essentially completely retained. (3) Alternative secondary structures, whether right- (A or C form) or left-handed (Z form), are not evident in either the packaged or unpackaged viral genome. (4) Small Raman hyperchromic effects (< 10%) observed for certain marker bands of dG, dA, and dT in the packaged state of P22 DNA suggest slightly reduced base-stacking interactions with packaging. These are consistent with previously reported UV hyperchromic effects, but the Raman spectrum shows that they are not associated with either base unpairing or strand separation.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriophages↗
PubMed1988

Translation initiation controls the relative rates of expression of the bacteriophage lambda late genes.

The late operon of bacteriophage lambda contains the genes encoding the morphogenetic proteins of the phage. These genes are transcribed equally from the single late promoter. Although the functional half-lives of the mRNA for the various genes of this operon vary less than 2-fold, their relative rates of expression have been shown to vary by nearly 1000-fold. This variation could result from differing rates of translation initiation, from overlapping upstream translation, or from differential elongation rates due to the presence of codons for which the corresponding tRNAs are rare. To distinguish between these possibilities, we have cloned sequences surrounding the initiator codons of several of these genes and measured their ability to drive synthesis of hybrid lambda-beta-galactosidase proteins. The rates of expression of the hybrid genes thus produced correlate very well with the natural rates of expression of the corresponding phage genes, suggesting that the rate of initiation of translation controls the relative expression rates of these genes.

Bacteriophage lambda↗
PubMed1974

Protein fusion: a novel reaction in bacteriophage lambda head assembly.

Parts of two phage-coded head proteins, pE and pC, become fused during bacteriophage lambda head assembly. pE is the main structural component of lambda heads and pC is a minor head protein that is not found as such in mature heads. The bond joining the two proteins appears to be covalent and is not a disulfide bond. Only a specific subset of the sequences of each protein is found in the fusion products, and these sequences are found in the products in equimolar amounts. Two nearly identical fusion products; X1 and X2, are detected; X2 is slightly smaller than X1 and appears to be a proteolytic cleavage product of X1. The fusion reaction probably takes place on a nascent head structure.

Autoradiography↗