We report 6 yr monitoring of a distant bright quasar CTS C30.10 ( z = 0.90052 ) with the Southern African Large Telescope ( SALT ) .
We measured the rest-frame time-lag of 562 \pm 2 days between the continuum variations and the response of the Mg II emission line , using the Javelin approach .
More conservative approach , based on five different methods , imply the time delay of 564 ^ { +109 } _ { -71 } days .
This time delay , combined with other available measurements of Mg II line delay , mostly for lower redshift sources , shows that the Mg II line reverberation implies a radius-luminosity relation very similar to the one based on a more frequently studied H \beta line .