I have some good news and some bad news for journalists who want to write mysteries.
The good news is, journalists seem to have a lot of luck getting agents. There must be the perception that "journalists can write" (a valid perception, I think) and can tell a story (after all, that's part of the craft).
Now, for the not-so-good news. I've been seeing a lot of first efforts lately by journalists whose manuscripts take the "journalist for a small-town newspaper who begins investigating a crime" tack. I haven't been buying any of these. It's not because they haven't been good. It's because I don't see these books as the future.
It breaks my heart to see what has become of newspapers and journalism in this country. I'm afraid that the newspaper is dying, though I do think (and have read research confirming) that people are more likely to read their small-town newspapers than a regional or national newspaper. Still, this sort of set-up is not exciting to me - it's all been done before, and I can't think of any recent small-town journalism series that have really taken off.
My advice is this: If you're going to have a journalist as your protagonist, you've got to do something modern with him or her. Have him report for a without-borders Internet news mag or Website. Make her a TV journalist -- a much better idea in a TV-saturated society. Have him traveling around doing something modern, like reporting on fashion from Milan or being disabled and blogging from home (it might be more interesting if the blog's readers didn't know about this handicap...) If the journalist covers food, make it less down-home and more reporting about the foodways of other nations, cultures, peoples. In short, make your journalist protag seem more on the cutting edge rather than a vestige of a dying industry.