America without JOHNNY?

It sounded unthinkable - yesterday, when news stories confirmed that Johnny Carson, 53, the longest running act in late-night TV history, was tired and planning to abdicate his throne and desk on NBC's "Tonight" show later this year.

Exit the king. Hail, Carnak. An institution, an American habit, coming to an end - but with much more of a bang than a winner.

Even as stock in RCA, which owns NBC, was dropping "fractionally" over the bad news, and even as an NBC spokesman was denouncing as "simply untrue" reports that Carson will bid adieu on Oct. 1, his 17th anniversary on the show, NBC News was desperately trying to land the NBC star for an interview to use on its own evening newscast.

"We got him, but we had to chase him halfway around Hollywood," said Paul Greenberg, executive producer of the "NBC Nightly News." "He didn't want to talk so they chased him to a tennis court." But neither NBC President Fred Silverman nor any other network executives would talk to NBC News, Greenberg said. "They think it's too sensitive; they don't want to make a statement."

Greenberg was particularly eager to get Carson on film because he knew CBS already had him - a camera crew from "60 Minutes," the top-rated news program in television, had been following Carson and associates for 10 days ending Monday. But his filming was for a profile of Carson scheduled to be seen next September when "60 Minutes" begins its new season - so Sanford Socolow, producer of "The CBS Evening News With Walter Cronkite," had to beseech Don Hewitt, producer of "60 Minutes," to part with some of his footage.

All this was negotiated yesterday while the film itself was still being processed in a lab.

"We've successfully negotiated a chunk of a priceless piece of conversation," Socolow proclaimed late yesterday afternoon as the film dried. He was asked what the bargaining tools were at in-house negotiations like this. "Screaming and yelling," he replied.

Carson himself called Mike Wallace, who couducted the interview, during the scramble over the film yesterday to ask if it would be on the Cronkite show. "He's such a funny guy," said Wallace admiringly. "In the interview I asked him about his successor. He said, 'Well, I hear Idi Amin's looking for work.' Today on the phone he said to me, 'Poor Howard K. Smith!'"

Howard K. Smith has announced he is leaving ABC's "World News Tonight" but that thunderbolt seems to have been totally eclipsed by the Carson announcement.

Much of the Wallace interview with Carson, conducted near a private plane ready to take off for Las Vegas, consisted of Wallace quoting from a conversation he'd had with Silverman. Carson sounded anything but warm and friendly toward the Silverman remarks being read to him.

When Wallace quoted Silverman as praising Carson's talent, Carson said sarcastically, "Awwww, told him Silverman called him an asset to NBC, Carson said, "An ass what?"

And when Wallace told Carson that, although Silverman was still at ABC when the new contract was negotiated, he had read it Carson feigned incredulity and said, "He actually reads?"

Meanwhile, NBC was trying to calm the turbulence or rumor by insisting there was no reason for alarm - which is a little like the Wizard of Oz saying, "Pay no attention to that man behind the curtain" when revealed as a humbug.

"This is truly a case of a media event, and we're hopeful that while the media event goes on, we will solve it," growled Executive Vice President M. S. Rukesyer Jr., in Burbank with Silverman. Silverman himself was incommunicado and had not even spoken to Carson since arriving on the Coast to look at prime-time pilots two weeks ago.

ABC's Barbara Walters, who can dial kings and prime ministers direct, was afraid yesterday she wouldn't be able to get Carson on the telephone, but she got right through (he had just returned to Los Angeles from Las Vegas) and said Carson told her he will leave the show not on Oct. 1, as speculated earlier, but sometime in the "late fall."

Walters asked Carson if he would still be on the show by the end of the year. Carson said, "No." NBC says Carson's current contract requires that he he continues as host of the "Tonight" show until the end of April 1981. Carson told Walters, "No one can force a performer to work if he doesn't fell he should."

"What surprised me," said Walters from her New York office, "is that Fred Silverman hasn't talked to Carson in a month, and Johnny remembers the exact date, March 17, of their last conversation. He said not one single NBC executive called to congratulate him after he hosted the Academy Awards."

David Brinkley introduced NBC's Carson segment, early on the "Nightly News" last night, by saying "Johnny Carson wanting to get off the 'Toninght' show is roughly like George Washington asking to get off the $1 bill."

"I think after 17 years, you can hardly be called a deserter," Carson told NBC News in the brief interview shown on the program. He declined to say exactly when he would leave the show and said it would be decided this weekend in talks with Silverman.

It was also pointed out on the NBC report that Carson's decision to leave could hardly come at a worse time for the network which has been "last in the prime-time ratings all but eight weeks since September."

Earlier, producer Greenberg had wondered aloud just how to play the Carson story.

"It may be the hottest story in town, but do we want to lead this instrument of public record with it or not?" Greenberg said. "What about the Rhodesian elections? What about the possible rubber strike? Is this Carson story too frivolous to lead a network news show?"

NBC ended up leading the "Nightly News" with the rubber story and economic news. But Carson was second. Wallace called the story "huge news" - at least "to the entertainment industry and NBC stockholders."

"60 Minutes" producer Hewitt said his plans to save the Carson profile until September might change, but he noted that there are only two more new "60 Minutes" telecast scheduled for this season; the program goes into reruns after the April 29 telecast. "The sets in use go down by a third the minute Daylight Savings Time kicks in," said Hewitt. "I'm not sure many people will be watching television at 7 o'clock Sunday nights."

Industry sources suggested yesterday that while NBC has faced the posibility of a Carson exodus, there are no known replacements now under consideration. Carson was "vague," Walters said, on the issue of whether he intended to stay at NBC until 1981 after leaving the "Tonight" show.

"What he didn't say," noted Walters, "was, 'Oh, no - I want to stay at NBC.' He said he didn't know."

Of course the way things are going, there may be some question whether NBC will be around, either.

The precise impact this bombshell of news was having on the American people could not be gauged yesterday. There was no rioting or loss of limb. Asked for his opinion of the significance of the Carson story as a news item, Socolow would only joke, "I get paid to give that lecture."

Carson is currently enjoying some of his 15 weeks of annual vacation from the "Tonight" show - one of the concessions he won in the contract he signed in December of 1977 - and is not scheduled to return to the program until May 2. Guest hosts will fill in until that time, and their ratings are always lower than Carson's. It remains to be seen what Carson will be able to accomplish as a lame-duck talk show host, once he makes public and definite the exact date of his departure.

Wsince the "Tonight" show represents an estimated total pretax profit od $122 million a year to NBC, the network is expected to resort to legal maneuvers, if necessary, to hold Carson to his contract - especially if, as industry specualtion has it, he responds to overtones reputedly coming his way from ABC. Rukeyser repeated yesterday the network's position that "We have a contract and that's where it's at."

And what of the little guy, or the little gal, the viewer on the street - orrather, in the bed - who has relied on Carson as a day's last source of dependable merriment since 1962? For at least one bereaved viewer, the loss was difficult to estimate.

For her, Carson represented more than a jester. One night she came out of the bathroom and heard Carson saying into the camera, "Did you remember to take your pill today?" As a matter of fact, she hadn't and she rushed back into the bathroom on Johnny's advice. CAPTION: Pictures 1, 2, 3, and 4, Johnny Carson: at left, clockwise from top, with a crowing champion rooster, in one of his costumes, and in 1954; right, Carson today.