The gene smu. 776 encodes a possible S-adenosylmethionine-dependent methyltransferase of 385 residues in Streptococcus mutans, a primary pathogen for human dental caries. The DNA fragment of smu.776 was cloned into pET28a and expressed in good amount from the E. coli strain BL21 (DE3). Smu.776 protein was purified to homogeneity in a two-step procedure ofNi2+ chelating and size exclusion chromatography. Crystals were obtained by hanging-drop vapor diffusion method and diffracted to 2.0 (A) resolution.The crystal belongs to orthorhombic space group C2 with cell dimension of a=168.47 (A), b= 50.66 (A), c=53.96 (A), β=104.22°. The asymmetric unit is expected to contain one molecule with solvent content of 51.3%.

Objective To conduct the calibration and comparative analysis on the Mindray BC 5800 automated hematology analy-zer in our department for ensuring the traceability and consistency of experimental results .Methods The mating calibrator of the Mindray company was used to conduct the calibration in the Mindray BC5800 automated hematology analyzer ,after calibration ,the calibration verification was performed .After passing the calibration verification ,the fresh blood was conduct the fixed value ,then the Mindray BC5800 and BC5180 automated hematology analyzers were calibrated ,after calibration ,the calibration verification was conducted ,after passing the calibration verification ,with the BC5800 as the standard instrument and BC5180 as the comparison in-strument ,the comparison experiment of fresh blood was performed .Results The Mindray BC5800 ,BC5300 and BC5180 automated hematology analyzers were passed the calibration verification after calibration ,in the comparison experiment of fresh blood ,the rela-tive deviation coincidence rates of WBC ,RBC ,Hb ,MCV ,PLT ,HCT ,MCH and MCHC between BC5800 and BC5300 and between BC5800 and BC5180 all exceeded 80% ,the comparison tests were passed .Conclusion The hematology analyzer should be per-formed the calibration verification and comparison test after calibration ,and the comparison test must be passed for ensuring the traceability of results and comparability among the test results in order to meet clinical needs .