Atara Biotherapeutics (ATRA) could be a solid addition to your portfolio given its recent upgrade to a Zacks Rank #2 (Buy). This rating change essentially reflects an upward trend in earnings estimates -- one of the most powerful forces impacting stock prices.A company's changing earnings picture is at the core of the Zacks rating. The system tracks the Zacks Consensus Estimate -- the consensus measure of EPS estimates from the sell-side analysts covering the stock -- for the current and following years.The ...