** Investment firm Alliance Trust's shares up 3.3pct, top FTSE mid-250 gainers, after RIT CapitalPartners makes informal merger proposal
** Alliance Trust says RIT has not provided any details ofproposal
** RIT confirms talks, says merger possibility at "a verypreliminary stage of consideration"; down 1.5 percent
** Alliance shares at highest in more than a year, just shyof record levels, and seeing their best day in 6 years
** Nearly a fifth of both stocks' daily avg gone through inthe first 15 minutes of trade; among most traded on FTSE 350 by avg vols (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)
