Horizontal DNA transfer mechanisms of bacteria as weapons of intragenomic conflict

Horizontal DNA transfer (HDT) is a pervasive mechanism of diversification in many microbial species, but its primary evolutionary role remains controversial. Much recent research has emphasised the adaptive benefit of acquiring novel DNA, but here we argue instead that intragenomic conflict provides a coherent framework for understanding the evolutionary origins of HDT. To test this hypothesis, we developed a mathematical model of a clonally descended bacterial population undergoing HDT through transmission of mobile genetic elements (MGEs) and genetic transformation. Including the known bias of transformation toward the acquisition of shorter alleles into the model suggested it could be an effective means of counteracting the spread of MGEs. Both constitutive and transient competence for transformation were found to provide an effective defence against parasitic MGEs; transient competence could also be effective at permitting the selective spread of MGEs conferring a benefit on their host bacterium. The coordination of transient competence with cell-cell killing, observed in multiple species, was found to result in synergistic blocking of MGE transmission through releasing genomic DNA for homologous recombination while simultaneously reducing horizontal MGE spread by lowering the local cell density. To evaluate the feasibility of the functions suggested by the modelling analysis, we analysed genomic data from longitudinal sampling of individuals carrying Streptococcus pneumoniae. This revealed the frequent within-host coexistence of clonally descended cells that differed in their MGE infection status, a necessary condition for the proposed mechanism to operate. Additionally, we found multiple examples of MGEs inhibiting transformation through integrative disruption of genes encoding the competence machinery across many species, providing evidence of an ongoing "arms race." Reduced rates of transformation have also been observed in cells infected by MGEs that reduce the concentration of extracellular DNA through secretion of DNases. Simulations predicted that either mechanism of limiting transformation would benefit individual MGEs, but also that this tactic's effectiveness was limited by competition with other MGEs coinfecting the same cell. A further observed behaviour we hypothesised to reduce elimination by transformation was MGE activation when cells become competent. Our model predicted that this response was effective at counteracting transformation independently of competing MGEs. Therefore, this framework is able to explain both common properties of MGEs, and the seemingly paradoxical bacterial behaviours of transformation and cell-cell killing within clonally related populations, as the consequences of intragenomic conflict between self-replicating chromosomes and parasitic MGEs. The antagonistic nature of the different mechanisms of HDT over short timescales means their contribution to bacterial evolution is likely to be substantially greater than previously appreciated

Reliability of code density test for high resolution ADCs

The effect of convertor internal noise on its output code histogram is analysed. Simulations show that large linearity errors, including missing codes, may vanish when sufficient noise is experienced. Consequently, the code density test is likely to yield a differential nonlinearity error within +/-0.5 LSB for any high accuracy noisy convertor.Anglai

A Cmos 13-b Cyclic Rsd A/d Converter

A 13-b CMOS cyclic A/D converter that does not need trimming nor digital calibration is presented. The effects associated with the error on the gain factor 2 as well as the offset errors are corrected by taking full advantage of the redundant signed digit (RSD) principle. The gain error resulting from mismatches among switched capacitors is corrected by a novel strategy that implements an exact multiplication by four after two cycles. As a result, offset errors do not affect the integral or the differential linearities from the RSD algorithm. The remaining overall shift caused by offsets is reduced under the LSB level by a proper choice of capacitor switching sequence. The converter achieves 1/2 LSB integral and differential linearity at 25 kS/s; harmonic distortion is less than -83 dB. Chip area is 2.9 mm2 in a standard CMOS 3-mu-m technology, including control logic and the serial-to-parallel output shift register. Power consumption is 45 mW under +/-5-V supplies