(Reuters) - Major bitcoin corporate backer MicroStrategy Inc said on Monday it bought another 328 bitcoin for about $15 million in cash, swelling its holdings of the cryptocurrency and sending the company’s shares 7% higher in premarket trading. The company, whose Chief Executive Officer Michael Saylor is one of the most vocal proponents of bitcoin, bought the cryptocurrency at an average price of about $45,710 each. MicroStrategy, the world"s largest publicly-traded business intelligence company, owns close to 90,859 bitcoin, which were acquired at an aggregate purchase price of about $2.19 billion and an average purchase price of around $24,063 per bitcoin, it said on Monday. bit.ly/2Mz4yj9 The company’s bitcoin holdings are valued at about $4.38 billion, according to a Reuters calculation. MicroStrategy spent last year steadily amassing bitcoin after making its first investment in August. The company had bought another 19,452 bitcoin last week, worth around $1.03 billion. MicroStrategy’s latest shopping spree comes as major firms including BNY Mellon, asset manager BlackRock Inc and credit card giant Mastercard Inc, backed certain cryptocurrencies, with Tesla Inc investing $1.5 billion in bitcoin. Bitcoin rose nearly 6% on Monday as risk assets rallied after last week’s bond rout cooled.