May the government use the speech of a public employee as the basis for discharge or denying promotion?

May the government use the speech of a public employee as the basis for discharge or denying promotion?

This is a politically charged topic, especially in today’s workplace.  Please investigate the following links.  Feel free to locate any other credible sources you may find on this subject for the benefit of your response:

http://www.pcwlawfirm.com/political-speech-the-workplace/

http://www.pcwlawfirm.com/first-amendment-freedom-speech-termination/

http://law2.umkc.edu/faculty/PROJECTS/FTRIALS/CONLAW/publicemployees.htm