One of the examples of Palestrina's ability to transform the melodies of plainsong into fresh and vital musical themes that entirely suit his own characteristic contrapuntal demands is the six-voice Missa de beata Virgine, published in his 1570 collection of masses titled Missarum liber tertius. The work is a paraphrase mass, taking chants from masses I, IX and XVII as laid out in the Liber Usualis collection of plainsong and weaving them into a newly composed polyphonic structure. The Missa de beata Virgine (which should not be confused with Palestrina's 1567 four-voice work by the same title), is so called because it employs liturgical melodies only used on the feast day of the same name.
The usual five major sections of the Mass Ordinary are all present. In the original version of the mass, Palestrina incorporated two Marian tropes -- "Mariam gubernans" and "Mariam coronans" -- into the text. The reform-minded Council of Trent, however, expunged both of these tropes from the official liturgy, and Palestrina, in a 1599 reissue, replaced the forbidden text with simple repetitions of the traditional mass texts around them.
The traditional Dorian-mode Kyrie chant for the Beata Virgine festival is in full six-voice imitation at the beginning of the movement. Four voices only are required for the following Christe. Palestrina cleverly combines the melodies of Kyrie I and Christe at the beginning of the second Kyrie: a three-layered contrapuntal structure built of two Christe motives and a single Kyrie motive is shifted about between the voices. Halfway through, a new fragment of plainchant sneaks in, undergoes imitative treatment, and then finds a home, in even, very chant-like whole notes, in the second tenor.
The Gloria is set, as per tradition, in two halves (divided at the text "Qui tollis peccata mundi"); during the second of these there is a brief but colorful change from binary to ternary rhythm. During the Credo, this same mechanism is employed to help convey the essence of the text "and rose again on the THIRD day"; the texture, appropriately, thins to three voices. The extensive Credo text is divided into three portions, each of which ends with a substantial cadence.
The Sanctus is divided into three (or, if you count the appended Benedictus, four) sections, alternating between six-and four-voice textures. Here the ancient cantus firmus technique is given new life, as the traditional Sanctus plainchant is given in whole and half notes (against an imitative background that sets the same basic intervalic material to much freer rhythmic values) by first the second tenor, then the first tenor (in the four-voice second section, beginning with the text "Plenisunt coeli"), and then the second tenor again as the six voices all cry "Hosanna in excelsis deo." During the Benedictus, the alto is given a chance to carry the cantus firmus. Having begun a cantus-firmus style, Palestrina is loathe to quit it, and during both the Agnus Dei I and Agnus Dei II the second tenor gives the traditional chant melody in long note-values.