Shares of J.C. Penney rose the most in six months after an analyst at ISI Group said the retailer could turn its top 300 stores into a real estate investment trust-like entity that would sublet space to other brands.Plano-based Penney rose 7 percent to $16.57 on the New York Stock Exchange, making it the biggest gainer in the Standard & Poor's 500 Index.A REIT operating under a separate name could be valued at about $40 a share, according to Omar Saad, an analyst at ISI Group. The remaining J.C. Penney-branded business, with 800 stores, could be worth about $6 a share, he wrote in a note Monday."JCP's most valuable asset is its low-cost real estate, and we believe there are many premium brands that would potentially be interested in subleasing space within the best locations," Saad wrote.Last month, J.C. Penney reported that annual sales plunged 25 percent to $13 billion, the lowest since at least 1987, during the first year of a transformation plan under CEO Ron Johnson. The department store company, which plans to turn most of the chain's stores into clusters of boutiques, has lost longtime J.C. Penney shoppers by experimenting with a shift to everyday low prices and replacing classic products with trendy new brands.Saad listed Ugg, H&M and Calvin Klein among brands and merchants that could be willing to lease space in the REIT unit. The remaining stores could continue under the J.C. Penney brand, he wrote.