You need to refer to the copyright notices on the relevant sites. For instance, prweb.com copyright notice states:
Vocus, Inc. and/or its subsidiary Vocus PRW Holdings LLC,
(collectively “PRWeb”) grant you a limited license to use and/or
republish any of the press releases on this site for any legitimate
media purpose, provided that you: (1) reference PRWeb as the source
and include a hyperlink to the original release on the PRWeb site; (2)
do not modify any press release wording; and (3) do not modify or add
hyperlinks, including but not limited to adlinks, within the press
This is just a snippet, you need to read it all.
In most cases you should be OK quoting a short passage and linking to the source material. However, I would refrain from "copy paste" entire articles - this is what hyperlinks are for.