Yes, you are, but you are expected to act in good faith in representing your employer's interests. In particular you are expected to be honest in your dealings on behalf of your employer, and not to profit from your position or from information that you personally have access to, or are aware of as a result of your job.
Having shares in other companies would really only present problems if it led to a conflict of interest, for example, if pursuit of personal financial gain meant you did not put your employer's interests first.
24 Feb 2012: When do you start getting paid?
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