<PAGE>
PROSPECTUS SUPPLEMENT DATED JANUARY 20, 1999
TO PROSPECTUS DATED MAY 8, 1998
$150,000,000
[LOGO]
6% CONVERTIBLE SUBORDINATED NOTES DUE 2005
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THIS PROSPECTUS SUPPLEMENT SHOULD BE READ IN CONJUNCTION WITH THE PROSPECTUS
DATED MAY 8, 1998 (THE "PROSPECTUS"). THE SELLING HOLDERS TABLE ON PAGE 66 OF
THE PROSPECTUS IS HEREBY SUPPLEMENTED TO INCLUDE ADDITIONAL NOTES NOW HELD BY A
CERTAIN SELLING HOLDER AND AN ADDITIONAL SELLING HOLDER AS INDICATED IN THE
FOLLOWING TABLE:
<TABLE>
<CAPTION>
NUMBER OF SHARES OF
PRINCIPAL COMMON STOCK
AMOUNT OF NOTES ISSUABLE UPON
OWNED AND THAT CONVERSION OF NOTES
NAME OF SELLING HOLDER MAY BE SOLD THAT MAY BE SOLD(1)
- ------------------------------------------------------------------------- --------------- ---------------------
<S> <C> <C>
NMS Services, Inc........................................................ $ 5,000,000(2) 176,366
NationsBanc Montgomery Securities........................................ $ 1,070,000(3) 37,742
</TABLE>
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(1) Includes only full shares of Common Stock issuable upon conversion of the
Notes based on an initial conversion price of $28.35 per share (initially
equivalent to a conversion price of 35,273 shares per $1,000 principal
amount of Notes). A cash payment will be made in lieu of any fractional
interest upon conversion.
(2) The holdings indicated above are the aggregate principal amount of Notes
held as of January 13, 1999.
(3) The holdings indicated above are additional Notes held as of January 13,
1999. NationsBanc Montgomery Securities has engaged in transactions with and
performed various investment banking and other services for the Company in
the past and may do so from time to time in the future.
The preceding table has been prepared based on information furnished to the
Company by or on behalf of the Selling Holders. With respect to each Selling
Holder identified in the Prospectus and each Prospectus Supplement, the
principal amount set forth may have increased or decreased since the information
was furnished, and there may be additional Selling Holders of which the Company
is unaware.
In view of the fact that Selling Holders may offer all or a portion of the
Notes or shares of Common Stock held by them pursuant to this offering, and
because this offering is not being underwritten on a firm commitment basis, no
estimate can be given as to the amount of Notes or the number of shares of
Common Stock that will be held by the Selling Holders after completion of this
offering. In addition, the Selling Holders identified in the Prospectus and each
Prospectus Supplement may have sold, transferred or otherwise disposed of all or
a portion of their Notes in transactions exempt from registration under the
Securities Act since the date on which they provided information regarding the
Notes.
