The most comprehensive list I’ve found so far on Twitter accounts for our Federal Level government officials…from the President down to each member of the House of Representatives.  Here are a few examples…

National Science Foundation: NSF
Environmental Protection Agency: EPA
Department of State: America.gov
Department of Health & Human Services: Food & Drug Administration: FDA Recalls
Department of Agriculture: Food Safety Information Center
U.S. Geological Survey: Earthquake & Tsunami Warning

For the whole list, visit:  http://twitter.pbworks.com/USGovernment