** Papa John's International PZZA.O shares advance ~5% at $34.50 premarket

** Firm's largest U.S. franchisee, controlling ~10% of its domestic restaurants, joins investment firm Irth Capital's bid to take the pizza chain private, Reuters reported citing sources

** Adds that Nadeem Bajwa, who operates nearly 300 Papa John's outlets, plans a "significant investment" in the buyout, with the board and management recently informed of his intentions

** PZZA has been in active discussions about a potential sale for the past year

** Has been reviewing Irth's offer, though no deal is guaranteed, report says

** Bajwa's interest strengthens Irth's $47-per-share offer made earlier this year backed by Brookfield Asset Management

** Irth, backed by a Qatari royal family member, owns about 10% of Papa John's, roughly half of which is in derivatives, the report said

** Up to last close, stock fell nearly 15% YTD