Between public relations practitioners and journalists, who defines news? Well, I'd say the journalists. PR practitioners may give the news to the journalists, mostly the good, but in the end, editors decide which news to publish. Usually, they tend to publish negative stories- a lot of the time these stories are more newsworthy. Let's be honest, who really wants to read a story that talks about how a business is running smoothly? Sure, it's good publicity for the business, but the public is more likely to read a story about some corruption or other controversy. One of the elements of newsworthiness is disaster, and disaster in business is definitely included.
However, although the journalists get to choose what makes publishing and what is left out, reporters still need PR practitioners in order to have this option. Without the practitioner, the journalists wouldn't get nearly as much news. Positive stories about businesses are usually easy to get because the business wants free publicity, so the reporters can just talk to them and get the stories easily.
However, journalists have to be careful about what the practitioner says- whether is puffed up or complete truth. Practitioners' job is to make their business look good, so their news isn't always objective and accurate. This gives a strain to their relationship with journalism even though the two need each other to get stories to the public.