<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): November 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                     (ON BEHALF OF PPLUS TRUST SERIES SPR-1)
             (Exact name of registrant as specified in its charter)

         DELAWARE             001-31912                    13-3891329
     (State or other        (Commission                (I. R. S. Employer
     jurisdiction  of        File Number)              Identification No.)
      incorporation)

  WORLD FINANCIAL CENTER,                                     10080
    NEW YORK, NEW YORK                                     (Zip Code)
   (Address of principal
    executive offices)

                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT

Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[ ] Written communications pursuant to Rule 425 under the Securities Act
    (17 CFR 230.425)

[ ] Soliciting material pursuant to Rule 14a-12 under the Exchange Act
    (17 CFR 240.14a-12)

[ ] Pre-commencement communications pursuant to Rule 14d-2(b) under the
    Exchange Act (17 CFR 240.14d-2(b))

[ ] Pre-commencement communications pursuant to Rule 13e-4(c) under the
    Exchange Act (17 CFR 240.13e-4(c))

<PAGE>

SECTION 1.        REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2.        FINANCIAL INFORMATION

                  Not applicable.

SECTION 3.        SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4.        MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5.        CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.

SECTION 6.        ASSET-BACKED SECURITIES

                  Not applicable.

SECTION 7.        REGULATION FD

                  Not applicable.

SECTION 8.        OTHER EVENTS

         ITEM 8.01         OTHER EVENTS

             99.1          Distribution to holders of the PPLUS Trust
                           Certificates Series SPR-1 on November 15, 2006.

                                    For information with respect to the
                           underlying securities held by PPLUS Trust Series
                           SPR-1, please refer to Sprint Nextel Corporation's
                           (Commission file number 001-04721) periodic reports,
                           including annual reports on Form 10-K, quarterly
                           reports on Form 10-Q and current reports on Form 8-K,
                           and other information on file with the Securities and
                           Exchange Commission (the "SEC"). You can read and
                           copy these reports and other information at the
                           public reference facilities maintained by the SEC at
                           Room 1580, 100 F Street, NE, Washington, D.C. 20549.
                           You may obtain copies of this material for a fee by
                           writing to the SEC's Public Reference Section of the
                           SEC at 100 F Street, NE, Washington, D.C. 20549. You
                           may obtain information about the operation of the
                           Public Reference Room by calling the SEC at
                           1-800-SEC-0330. You can also access some of this
                           information electronically by means of the SEC's
                           website on the Internet at http://www.sec.gov, which
                           contains reports, proxy and information statements
                           and other information that the underlying securities
                           issuer has filed electronically with the SEC.
<PAGE>

                                    Although we have no reason to believe the
                           information concerning the underlying securities, the
                           underlying guarantee, the underlying securities
                           issuer or the underlying securities guarantor
                           contained in the underlying securities guarantor's
                           Exchange Act reports is not reliable, neither the
                           depositor nor the trustee participated in the
                           preparation of such documents or made any due
                           diligence inquiry with respect to the information
                           provided therein. No investigation with respect to
                           the underlying securities issuer or the underlying
                           securities guarantor (including, without limitation,
                           no investigation as to their financial condition or
                           creditworthiness) or of the underlying securities or
                           the underlying guarantee has been made. You should
                           obtain and evaluate the same information concerning
                           the underlying securities issuer and the underlying
                           securities guarantor as you would obtain and evaluate
                           if your investment were directly in the underlying
                           securities or in other securities issued by the
                           underlying securities issuer or the underlying
                           securities guarantor. There can be no assurance that
                           events affecting the underlying securities and the
                           underlying guarantee or the underlying securities
                           issuer and the underlying securities guarantor have
                           not occurred or have not yet been publicly disclosed
                           which would affect the accuracy or completeness of
                           the publicly available documents described above.

SECTION 9.        FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01         FINANCIAL STATEMENTS AND EXHIBITS

                  (a)      Financial statements of business acquired.

                           Not Applicable.

                  (b)      Pro forma financial information.

                           Not Applicable.

                  (c)      Shell company transactions.

                           Not Applicable.

                  (d)      Exhibits.

                             99.1   Trustee's report in respect of
                                    the November 15, 2006 distribution to
                                    holders of the PPLUS Trust Certificates
                                    Series SPR-1.
<PAGE>

                                   SIGNATURES

                  Pursuant to the requirements of the Securities Exchange Act of
                  1934, the registrant has duly caused this report to be signed
                  on its behalf by the undersigned, hereunto duly authorized.


                                      MERRILL LYNCH DEPOSITOR, INC.

         Date: 11/16/2006             By:     /s/ Jason Liddell
                                              ---------------------------
                                      Name:  Jason Liddell
                                      Title: Vice-President


<PAGE>

                                  EXHIBIT INDEX


                99.1       Trustee's report in respect of the November 15,
                           2006 distribution to holders of the PPLUS Trust
                           Certificates Series SPR-1.
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SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): November 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                     (ON BEHALF OF PPLUS TRUST SERIES SPR-1)
             (Exact name of registrant as specified in its charter)

         DELAWARE             001-31912                    13-3891329
     (State or other        (Commission                (I. R. S. Employer
     jurisdiction  of        File Number)              Identification No.)
      incorporation)

  WORLD FINANCIAL CENTER,                                     10080
    NEW YORK, NEW YORK                                     (Zip Code)
   (Address of principal
    executive offices)

                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT

Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[ ] Written communications pursuant to Rule 425 under the Securities Act
    (17 CFR 230.425)

[ ] Soliciting material pursuant to Rule 14a-12 under the Exchange Act
    (17 CFR 240.14a-12)

[ ] Pre-commencement communications pursuant to Rule 14d-2(b) under the
    Exchange Act (17 CFR 240.14d-2(b))

[ ] Pre-commencement communications pursuant to Rule 13e-4(c) under the
    Exchange Act (17 CFR 240.13e-4(c))
SECTION 1.        REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2.        FINANCIAL INFORMATION

                  Not applicable.

SECTION 3.        SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4.        MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5.        CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.

SECTION 6.        ASSET-BACKED SECURITIES

                  Not applicable.

SECTION 7.        REGULATION FD

                  Not applicable.

SECTION 8.        OTHER EVENTS

         ITEM 8.01         OTHER EVENTS

             99.1          Distribution to holders of the PPLUS Trust
                           Certificates Series SPR-1 on November 15, 2006.

                                    For information with respect to the
                           underlying securities held by PPLUS Trust Series
                           SPR-1, please refer to Sprint Nextel Corporation's
                           (Commission file number 001-04721) periodic reports,
                           including annual reports on Form 10-K, quarterly
                           reports on Form 10-Q and current reports on Form 8-K,
                           and other information on file with the Securities and
                           Exchange Commission (the "SEC"). You can read and
                           copy these reports and other information at the
                           public reference facilities maintained by the SEC at
                           Room 1580, 100 F Street, NE, Washington, D.C. 20549.
                           You may obtain copies of this material for a fee by
                           writing to the SEC's Public Reference Section of the
                           SEC at 100 F Street, NE, Washington, D.C. 20549. You
                           may obtain information about the operation of the
                           Public Reference Room by calling the SEC at
                           1-800-SEC-0330. You can also access some of this
                           information electronically by means of the SEC's
                           website on the Internet at http://www.sec.gov, which
                           contains reports, proxy and information statements
                           and other information that the underlying securities
                           issuer has filed electronically with the SEC.
Although we have no reason to believe the
                           information concerning the underlying securities, the
                           underlying guarantee, the underlying securities
                           issuer or the underlying securities guarantor
                           contained in the underlying securities guarantor's
                           Exchange Act reports is not reliable, neither the
                           depositor nor the trustee participated in the
                           preparation of such documents or made any due
                           diligence inquiry with respect to the information
                           provided therein. No investigation with respect to
                           the underlying securities issuer or the underlying
                           securities guarantor (including, without limitation,
                           no investigation as to their financial condition or
                           creditworthiness) or of the underlying securities or
                           the underlying guarantee has been made. You should
                           obtain and evaluate the same information concerning
                           the underlying securities issuer and the underlying
                           securities guarantor as you would obtain and evaluate
                           if your investment were directly in the underlying
                           securities or in other securities issued by the
                           underlying securities issuer or the underlying
                           securities guarantor. There can be no assurance that
                           events affecting the underlying securities and the
                           underlying guarantee or the underlying securities
                           issuer and the underlying securities guarantor have
                           not occurred or have not yet been publicly disclosed
                           which would affect the accuracy or completeness of
                           the publicly available documents described above.

SECTION 9.        FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01         FINANCIAL STATEMENTS AND EXHIBITS

                  (a)      Financial statements of business acquired.

                           Not Applicable.

                  (b)      Pro forma financial information.

                           Not Applicable.

                  (c)      Shell company transactions.

                           Not Applicable.

                  (d)      Exhibits.

                             99.1   Trustee's report in respect of
                                    the November 15, 2006 distribution to
                                    holders of the PPLUS Trust Certificates
                                    Series SPR-1.
SIGNATURES

                  Pursuant to the requirements of the Securities Exchange Act of
                  1934, the registrant has duly caused this report to be signed
                  on its behalf by the undersigned, hereunto duly authorized.

                                      MERRILL LYNCH DEPOSITOR, INC.

         Date: 11/16/2006             By:     /s/ Jason Liddell
                                              ---------------------------
                                      Name:  Jason Liddell
                                      Title: Vice-President
EXHIBIT INDEX

                99.1       Trustee's report in respect of the November 15,
                           2006 distribution to holders of the PPLUS Trust
                           Certificates Series SPR-1.