Found it.
The article was published in E! Online on May 11, 1999 following a press conference with George Lucas and contains a more extensive quote (reproduced in its entirety below). Note that the author has transcribed midi-chlorians incorrectly, referring to them as midichondrians.
Will you ever see the movie with an audience?
I probably will at some point, but it'll be later in the summer. It's
inevitable that one wants to do that, see what real people think about
it.
Is there something about you that exposure in the media has changed, 60 Minutes and so forth?
Again, I've been through these hoops a lot. Whenever a movie comes
out, whether it's Star Wars or Howard the Duck, I end up on a lot of
covers, and I end up with a lot of media presence for a period of
time--with Howard the Duck, it was about 10 minutes, with Star Wars I
expect it to be about three or four months. I'm assuming by fall
things will have calmed down again, and I can go back to not having
this much media attention. The great thing about living in San
Francisco is it's a great cosmopolitan city, but you don't have a lot
of people chasing you around all the time to get autographs and
things. Every once in a while somebody comes up, but it's usually a
little kid. It's a sophisticated environment. It's easier to live in.
I know you're saying Saturday-morning serial for kids, but I saw in this movie you're going from classic, heroic stuff into much more
specifically Judeo-Christian ideas. Can you talk a little about where
you see the meaning of the Star Wars movies going?
Well, I'm confused. I've been very conscious about not making the
movies religion specific. When I brought the first Star Wars movie
out, it seemed most religions used it to demonstrate their own
beliefs--not only Judeo-Christian but also Eastern. And in many cases,
people have said they're much more Eastern than Western in nature. I'm
a little curious as to why you decided this one was a little more like
that.
The ideas of virgin birth, the freeing of slaves...
Did you see Spartacus?
That's Western, too.
Yes, that's Western, too. Actually the same thing has happened in
China and in India, especially. The issue of Immaculate Conception is
a motif that runs through all religions. It's a motif that runs
through all stories about the local deity or the local hero. It's the
same thing with Hercules. Most heroes are conceived in an unusual way.
And in this particular case, it's actually not Immaculate Conception,
it's conception by metaphor, I would say, more than anything else. I'm
taking the idea of symbiotic relationships and trying to demonstrate
it in a more concrete way. Midichondrians are a loose depiction of
mitochondria, which are necessary components for cells to divide. They
probably had something--which will come out someday--to do with the
beginnings of life and how one cell decided to become two cells with a
little help from this other little creature who came in, without whom
life couldn't exist. And it's really a way of saying we have hundreds
of little creatures who live on us, and without them, we all would
die. There wouldn't be any life.They are necessary for us; we are
necessary for them. Using them in the metaphor, saying society is the
same way, says we all must get along with each other. The planet is
the same way. We must treat the other creatures on the planet with
respect, otherwise the planet will die.
E! Online - From the Star Wars Flu to His Beloved Pit Droids, George Lucas Talks Phantom Phenom
The interview was also carried in the August edition of Starlog #265. Note that the quote stops with a period in the Starlog article (downloadable here) but continues with a comma and then the stuff about midi-chlorians in the article above. This wasn't a one-on-one interview but a press-conference (in which Starlog were in attendance) which would explain the difference in transcription.
