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. 

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. 

(Reporting by Noor Zainab Hussain in Bengaluru; Editing by Krishna Chandra Eluri) ((noor.hussain@thomsonreuters.com; Within U.S. +1 646 223 8780; Outside U.S. +91 80 6182 2663 or +91 80 3796 2663 ;))