Summary. Insertion mutagenesis with the help of the plasmid pFS23 was used to generate Yersinia pestis fra mutants. The results of pFra-strain production under non-selective conditions suggested that such Y . pestis variants may be generated in natural plague foci at high frequency and may participate in supporting the epizootic process. Present data suggest that the reduction of virulence in Fra-strains reported by the majority of investigators was connected with the use of Y. pestis variants carrying additional unidentified mutations. It was shown that the loss of the ability to produce capsular antigen (FI) alone or in combination with absence of murine toxin production did not lead to an increase in LD50 absolute values. Simultaneous loss of these two virulence determinants did not influence the duration of survival of the infected animals. However, absence of only FI antigen production in the infecting strain resulted in prolonged survival of the infected animals. Conversion of plague infection from acute to chronic form is probably dependent on the animal host species and the Y. pestis parent strain subjected to mutagenesis.

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The article describes a scanning polarization lidar, LOSA-M3, developed at the V. E. Zuev Institute of Atmospheric Optics, the Siberian Branch of the Russian Academy of Sciences (IAO SB RAS), as part of the common use center "Atmosphere". The first results of studying the crystalline particle orientation by means of this lidar are presented herein. The main features of the LOSA-M3 lidar are the following: (1) an automatic scanning device, which allows changing the sensing direction in the upper hemisphere at the speed up to 1.5 • s −1 with the accuracy of the angle measurement setting of at least 1 arcmin, (2) separation of the polarization components of the received radiation that is carried out directly behind the receiving telescope without installing the elements distorting polarization, such as dichroic mirrors and beam splitters, and (3) continuous alternation of the initial polarization state (linear-circular) from pulse to pulse that makes it possible to evaluate some elements of the scattering matrix.For testing lidar performance several series of measurements of the ice cloud structure in the zenith scan mode were carried out in Tomsk in April-June 2018. The results show that the degree of horizontal orientation of particles can vary significantly in different parts of the cloud. The dependence of signal intensity on the tilt angle reflects the distribution of particle deflection relative to the horizontal plane and is well described by the exponential dependence. The values of the cross-polarized component in most cases show a weak decline of intensity with the angle. However, these variations are smaller than the measurement errors. We can conclude that they are practically independent of the tilt angle. In most cases the scattering intensity at the wavelength of 532 nm has a wider distribution than at 1064 nm.