An analysis of authorship attribution: Identifying linguistic variables in oral and written discourse

This dissertation goes into the new field from applied linguistics called forensic linguistics, which studies the language as an evidence for criminal cases. There are many subfields within forensic linguistics, however, this study belongs to authorship attribution analysis, where the authorship of a text is attributed to an author through an exhaustive linguistic analysis. Within this field, this study analyzes the morphosyntactic and discursive-pragmatic variables that remain constant in the intra-variation or personal style of a speaker in the oral and written discourse, and at the same time have a high difference rate in the interspeaker variation, or from one speaker to another. The theoretical base of this study is the term used by professor Maria Teresa Turell called “idiolectal style”. This term establishes that the idiosyncratic choices that the speaker makes from the language build a style for each speaker that is constant in the intravariation of the speaker’s discourse. This study comes as a consequence of the problem appeared in authorship attribution analysis, where the absence of some known texts impedes the analysis for the attribution of the authorship of an uknown text. Thus, through a methodology based on qualitative analysis, where the variables are studied exhaustively, and on quantitative analysis, where the findings from qualitative analysis are statistically studied, some conclusions on the evidence of such variables in both oral and written discourses will be drawn. The results of this analysis will lead to further implications on deeper analyses where larger amount of data can be used.

Fitness cost and interference of Arm/Rmt aminoglycoside resistance with the RsmF housekeeping methyltransferases

Arm/Rmt methyltransferases have emerged recently in pathogenic bacteria as enzymes that confer high-level resistance to 4,6-disubstituted aminoglycosides through methylation of the G1405 residue in the 16S rRNA (like ArmA and RmtA to -E). In prokaryotes, nucleotide methylations are the most common type of rRNA modification, and they are introduced posttranscriptionally by a variety of site-specific housekeeping enzymes to optimize ribosomal function. Here we show that while the aminoglycoside resistance methyltransferase RmtC methylates G1405, it impedes methylation of the housekeeping methyltransferase RsmF at position C1407, a nucleotide that, like G1405, forms part of the aminoglycoside binding pocket of the 16S rRNA. To understand the origin and consequences of this phenomenon, we constructed a series of in-frame knockout and knock-in mutants of Escherichia coli, corresponding to the genotypes rsmF(+), ΔrsmF, rsmF(+) rmtC(+), and ΔrsmF rmtC(+). When analyzed for the antimicrobial resistance pattern, the ΔrsmF bacteria had a decreased susceptibility to aminoglycosides, including 4,6- and 4,5-deoxystreptamine aminoglycosides, showing that the housekeeping methylation at C1407 is involved in intrinsic aminoglycoside susceptibility in E. coli. Competition experiments between the isogenic E. coli strains showed that, contrary to expectation, acquisition of rmtC does not entail a fitness cost for the bacterium. Finally, matrix-assisted laser desorption ionization (MALDI) mass spectrometry allowed us to determine that RmtC methylates the G1405 residue not only in presence but also in the absence of aminoglycoside antibiotics. Thus, the coupling between housekeeping and acquired methyltransferases subverts the methylation architecture of the 16S rRNA but elicits Arm/Rmt methyltransferases to be selected and retained, posing an important threat to the usefulness of aminoglycosides worldwide.