BUZZ-Alliance Trust: RIT Capital makes informal merger proposal
** Investment firm Alliance Trust's shares up 3.3 pct, top FTSE mid-250 gainers, after RIT Capital Partners (LSE: 0QOQ.L - news) makes informal merger proposal
** Alliance Trust says RIT has not provided any details of proposal
** RIT confirms talks, says merger possibility at "a very preliminary stage of consideration"; down 1.5 percent
** Alliance shares at highest in more than a year, just shy of record levels, and seeing their best day in 6 years
** Nearly a fifth of both stocks' daily avg gone through in the first 15 minutes of trade; among most traded on FTSE 350 by avg vols (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)