** Shares of drugmaker Atara Biotherapeutics ATRA.O fall 49% to $6.91 premarket
** Co says U.S. FDA has rejected its application to market Ebvallo, a treatment for a rare and aggressive cancer linked to the Epstein-Barr virus in patients who have received organ transplants
** FDA said main trial was insufficient to prove effectiveness; had earlier indicated single-arm trial was acceptable - ATRA
** No safety or manufacturing concerns raised, co says; plans meeting with FDA to discuss next steps
** Epstein-Barr virus is a common virus that can cause infections and, in rare cases, lead to certain cancers
** As of last close, stock up ~36% over the past year
(Reporting by Sahil Pandey in Bengaluru)
((Sahil.Pandey@thomsonreuters.com))