SAO PAULO, Jan 5 (Reuters) - Brazilian investment firm MAM Asset Management, controlled by tycoon Nelson Tanure, said late on Tuesday it plans to make a public acquisition offer for 100% of medical diagnostics company Alliar (AALR3.SA) if it reaches a controlling stake in the firm. Current controlling shareholders of Alliar, formally known as Centro de Imagem Diagnosticos SA (AALR3.SA), have agreed to sell up to 52.75% of the company to Tanure, but its bylaws determine that any investor to hold a stake of more than 50% is required to present a takeover bid for the whole firm. Despite the 52.75% threshold, Tanure"s investment firm noted in a letter that some of the current controlling shareholders will be allowed to remain in the company under the deal, implying the poison pill could not be triggered. Minority shareholders have expressed discontent with the move, as Tanure would not be necessarily required to also bid for their stakes. "A substantial part of the current controlling shareholders is formed by doctors who founded Alliar or companies acquired by it. They have expertise in the industry and thus add value to Alliar," MAM said - though adding they will have the option to sell their stakes to the fund in the future. "It is certain that if the amount of stocks acquired by the fund implies changing the company"s control, the fund will immediately present a public acquisition offer as required by the rules," it said. Alliar had previously announced MAM will pay the controlling block 20.50 reais ($3.61) per share in the company, whose stocks closed at 13.51 reais on Tuesday. ($1 = 5.6770 reais)